We have pretty reliable maps of the Lyran Commonwealth both before and after the Archon's "police action" snapping up what Kerensky left behind on his way to Terra.
Here it is as of 2571 (founding of the Star League):
https://www.sarna.net/wiki/File:Lc-2571.pngFor perspective here is a map of the Inner Sphere as a whole as of 2750:
https://www.sarna.net/wiki/File:2750_Inner_Sphere_(Sarna).svg If we're just going by eyeballing the "area" of these very irregular shapes, then we can see the LC is probably about equal to the Combine and FedSuns, a little bigger than the FWL, and markedly bigger than the Confederation, and MUCH bigger than the Hegemony.
(Of course, most of that "area" is empty space, what matters is the number of settled systems within it, but aside from the Confederation being noticeably more "tightly packed" with systems, which is interesting but off-topic, none of the factions look significantly more "dense" than others so we can use area within borders as a decent approximation of meaningful "size.")
And here is the LC after the 1st SW. There are also changes that happened along the Kurita and Marik borders here of course, but we can ignore that, the shading shows the change in the Periphery rim border:
https://www.sarna.net/wiki/File:Lc-1sw-2822.pngActually, there is a better version on the RWR page:
https://www.sarna.net/wiki/File:Rwr_-_first_sw.jpg To save the time of eyeballing the map, Sarna also has a list of former RWR systems absorbed (although the usual disclaimers about Sarna not being 100% reliable apply):
https://www.sarna.net/wiki/Republic-Commonwealth_War It comes to 94 worlds.
In sum: the Lyran Commonwealth was never small per se, but it did get bigger. Nevertheless, it should be noted that the Commonwealth's power has always been its economic and industrial might. I feel like there may be some kind of idea that because the Commonwealth is known for having lots of factories producing lots of big 'Mechs, everything about it is "big," but it's never been the biggest Successor State by sheer number of worlds. That "honor" actually goes to the FedSuns.
As for the RWR... if you go to Sarna's page for
former RWR systems and just click on a few names at random, what you'll see over and over again is that the worlds that DIDN'T become Lyran disappear from the map entirely, with some becoming parts of pirate havens or minor Periphery states.