I'm one of the devs, I need to know this type of things lol
Seriously though, we don't have official guides with tables listing each of the sizes implemented in game yet (as the game is still WIP), but for main routes their sizes tends to follow this pattern:
001 - 009 is Size 1; 010 - 029 is Size 2; 030 - 049 is Size 3; 050 - 069 is Size 4; 070 - 089 is Size 5; 090 - 100 is Size 6
These numbers are arbitrary though and can differ between routes with plus / minus a few scenes as while 100 scenes is a minimum target for each main route, some of them will have more than 100 scenes. Optional events will have much smaller scene count, so their numbers for each of the sizes may differ
As for MC scene you listed, the unlock condition for MC012 is finishing global032 (a mandatory event informing players about current Naomi's predicament) and this one is very early size 3 scene. MC012 uses some story beats from this scene as a background for its story