I didn't have a particular length in mind, but from glancing through designs with similar weights, 800 metres would be pretty typical. Given this is a broadside-preferring ship, and it's so festooned with grav decks, it's probably a bit longer for its weight than most. Let's say 850 metres.
I've changed the design as you said. HNPPCs are now LNPPCs, the double Gauss has been added to the bow, and the double NL-35s have been upgraded to quads. (You said triples, but the extra mass freed up by the PPC downgrade was enough for quads). The endurance has also been increased - the crew all get first-class quarters now(which was one I wanted in the original design but couldn't squeeze in), and the auxiliary fuel tank is 50% bigger, up to 15,000 tons total. Defensive weapons have been changed from 6 ERPPC + 6 LPL + 8 AMS per facing to 4 ERLL + 3 Gauss + 3 UAC/20 + 4 AMS per facing. That's a bit more raw firepower, but it won't hit quite as hard in practice, and the biggest guns(the UAC's) have fairly short ranges. Because of the reduced heat loads all around, the heat profile of the ship is substantially improved as well - it can now fire ~3/4 of its weapons each turn, instead of ~1/2.
Class/Model/Name: Bagration II
Tech: Inner Sphere
Ship Cost: $37,091,937,120.00
Magazine Cost: $23,640,000.00
BV2: 80,485
Mass: 770,000
K-F Drive System: Compact with L-F Battery
Power Plant: Maneuvering Drive
Safe Thrust: 3
Maximum Thrust: 5
Armor Type: Lamellor Ferro-Carbide
Armament:
32 AMS (C)
16 Naval PPC Light
4 Naval AC 30
36 Naval Laser 35
2 Naval Gauss Rifle Medium
32 Laser Large ER (C)
24 Gauss Rifle (C)
24 AC 20 Ultra (C)
Class/Model/Name: Bagration II
Mass: 770,000
Equipment: Mass
Drive: 138,600
Thrust
Safe: 3
Maximum: 5
Controls: 1,925
K-F Hyperdrive: Compact with L-F Battery (17 Integrity) 356,125
Jump Sail: (5 Integrity) 69
Structural Integrity: 75 57,750
Total Heat Sinks: 1811(3622) Double 1,240
Fuel & Fuel Pumps: 11250 points 4,590
Fire Control Computers: 0
Armor: 1155 pts Lamellor Ferro-Carbide 1,155
Fore: 200
Fore-Left/Right: 200/200
Aft-Left/Right: 200/200
Aft: 155
Dropship Capacity: 10 10,000
Grav Decks:
Small: 4 200
Medium: 3 300
Large: 2 1,000
Escape Pods: 30 210
Life Boats: 30 210
Crew And Passengers:
50 Officers in 1st Class Quarters 500
149 Crew in 1st Class Quarters 1,490
94 Gunners and Others in 1st Class Quarters 940
100 Bay Personnel 0
# Weapons Loc Heat Damage Range Mass
4 AMS (C) Nose 4 12 (1.2-C) PDS 2
4 AMS (C) RBS 4 12 (1.2-C) PDS 2
4 AMS (C) Aft 4 12 (1.2-C) PDS 2
4 AMS (C) LBS 4 12 (1.2-C) PDS 2
4 AMS (C) FR 4 12 (1.2-C) PDS 2
4 AMS (C) FL 4 12 (1.2-C) PDS 2
4 AMS (C) AR 4 12 (1.2-C) PDS 2
4 AMS (C) AL 4 12 (1.2-C) PDS 2
4 Naval PPC Light FR 420 280 (28-C) Long-C 5,600
4 Naval PPC Light FL 420 280 (28-C) Long-C 5,600
4 Naval PPC Light AR 420 280 (28-C) Long-C 5,600
4 Naval PPC Light AL 420 280 (28-C) Long-C 5,600
2 Naval AC 30 LBS 200 600 (60-C) Long-C 7,000
2 Naval AC 30 RBS 200 600 (60-C) Long-C 7,000
8 Naval Laser 35 Nose 416 280 (28-C) Long-C 5,600
4 Naval Laser 35 Aft 208 140 (14-C) Long-C 2,800
4 Naval Laser 35 RBS 208 140 (14-C) Long-C 2,800
4 Naval Laser 35 LBS 208 140 (14-C) Long-C 2,800
4 Naval Laser 35 FR 208 140 (14-C) Long-C 2,800
4 Naval Laser 35 FL 208 140 (14-C) Long-C 2,800
4 Naval Laser 35 AR 208 140 (14-C) Long-C 2,800
4 Naval Laser 35 AL 208 140 (14-C) Long-C 2,800
2 Naval Gauss Rifle Medium Nose 30 500 (50-C) Extreme-C 11,000
4 Laser Large ER (C) Nose 48 40 (4-C) Extreme 16
4 Laser Large ER (C) RBS 48 40 (4-C) Extreme 16
4 Laser Large ER (C) Aft 48 40 (4-C) Extreme 16
4 Laser Large ER (C) LBS 48 40 (4-C) Extreme 16
4 Laser Large ER (C) FR 48 40 (4-C) Extreme 16
4 Laser Large ER (C) FL 48 40 (4-C) Extreme 16
4 Laser Large ER (C) AR 48 40 (4-C) Extreme 16
4 Laser Large ER (C) AL 48 40 (4-C) Extreme 16
3 Gauss Rifle (C) Nose 3 45 (4.5-C) Long 36
3 Gauss Rifle (C) Aft 3 45 (4.5-C) Long 36
3 Gauss Rifle (C) RBS 3 45 (4.5-C) Long 36
3 Gauss Rifle (C) LBS 3 45 (4.5-C) Long 36
3 Gauss Rifle (C) FR 3 45 (4.5-C) Long 36
3 Gauss Rifle (C) FL 3 45 (4.5-C) Long 36
3 Gauss Rifle (C) AR 3 45 (4.5-C) Long 36
3 Gauss Rifle (C) AL 3 45 (4.5-C) Long 36
3 AC 20 Ultra (C) Nose 42 90 (9-C) Medium 36
3 AC 20 Ultra (C) Aft 42 90 (9-C) Medium 36
3 AC 20 Ultra (C) RBS 42 90 (9-C) Medium 36
3 AC 20 Ultra (C) LBS 42 90 (9-C) Medium 36
3 AC 20 Ultra (C) FR 42 90 (9-C) Medium 36
3 AC 20 Ultra (C) FL 42 90 (9-C) Medium 36
3 AC 20 Ultra (C) AR 42.00 90 (9-C) Medium 36
3 AC 20 Ultra (C) AL 42 90 (9-C) Medium 36
Ammo Rounds Mass
AMS (C) Ammo 192 8.00
AMS (C) Ammo 192 8.00
AMS (C) Ammo 192 8.00
AMS (C) Ammo 192 8.00
AMS (C) Ammo 192 8.00
AMS (C) Ammo 192 8.00
AMS (C) Ammo 192 8.00
AMS (C) Ammo 192 8.00
Naval AC 30 Ammo 100 80.00
Naval AC 30 Ammo 100 80.00
Naval Gauss Rifle Medium Ammo 100 40.00
Gauss Rifle (C) Ammo 120 15.00
Gauss Rifle (C) Ammo 120 15.00
Gauss Rifle (C) Ammo 120 15.00
Gauss Rifle (C) Ammo 120 15.00
Gauss Rifle (C) Ammo 120 15.00
Gauss Rifle (C) Ammo 120 15.00
Gauss Rifle (C) Ammo 120 15.00
Gauss Rifle (C) Ammo 120 15.00
AC 20 Ultra (C) Ammo 120 24.00
AC 20 Ultra (C) Ammo 120 24.00
AC 20 Ultra (C) Ammo 120 24.00
AC 20 Ultra (C) Ammo 120 24.00
AC 20 Ultra (C) Ammo 120 24.00
AC 20 Ultra (C) Ammo 120 24.00
AC 20 Ultra (C) Ammo 120 24.00
AC 20 Ultra (C) Ammo 120 24.00
Number Equipment and Bays Mass
20 Bay Fighter 3,000
6 Bay Small Craft 1,200
100,000 Cargo, Standard 100,000
13,650 Cargo, Liquid 15,000
1 NCSS Large 500
1 Mobile HPG 50
5 Bay Battle Armor 5 50
(I'm not sure if the unity of the defensive armaments bugs you or not - I tend to like it, because it seems easier to play, easier to grasp the TRO mentally, and because it's a good way to keep myself from min-maxing. But if you want me to get a bit weirder, I can. I was kicking around half a dozen other possible weapon types before I settled on those, so I can mix it up easily enough)