just out of curiosity, what makes this Apollo 7 as opposed to 5, 11, 13, etc?

Apollo 7 was I believe a Saturn IB which is a smaller cousin to the Saturn V. 5 was unamnned and 11 went to the moon land landed but 13 had a explosion at its oxygen tanks thanks to faulty wiring so they had to do a return trajectory.

it's difficult to determine the size of the craft, especially if trying to compare it to another...

It's listed as Apollo 7 by the artist.

Artist said:
アポロ7号with射命丸
アポロ計画のなか初めて有人飛行をしたロケットですね。・・・たぶん　ロケットは模写でかなり適当です。　一方地球は完全にオリジナルです

"Apollo 7 with Shameimaru
This was the first manned flight of the Apollo Project. ...I think. I traced the rocket so it's pretty reasonable. On the other hand the Earth is entirely original."