You could look at average CAT of the party and do that CAT or CAT-1.
Additionally, you could credit a certain amount of XP and scrip based on average of the party or average of mission performance so far.
Alternatively you could work out expected XP/scrip per completed mission and scale that to the campaign. For example, you might expect that a character will at least survive, do one agenda item, and take one injury per mission. That gives 3 xp per mission. For scrip, let's assume they make 3-7 scrip per mission on average due to body retrieval, mission outcome, etc. That's a solid pool of resources they can convert into a good new character sheet.
I think that generally size of sin scales with CAT. obviously subject to Admin discretion, so you can definitely have a fly-sized cat-7 sin.
But if you have a cat-7 sin with the size of a fly it will still have durability commensurate with CAT. considering cat-7 is durable enough to make a cat-4 attack (size/destruction on par with a large vehicle) ineffective it seems that no cat-1 attack, regardless of relative size, will be effective. You could play it as strength of psychic phenomena.
Going from the media that inspired CAIN, for example JJK: Gojo (arguably cat-7) can no-sell / tank any sort of attack because it can't hit him or make an impact on his Limitless defenses. It doesn't matter if a cat-1 attack is bigger than him, since his phenomena make it impossible to make a lasting impact in the first place.
A little rambling, but hope this helps. The lack of hard rules wrt power scaling make CAIN really interesting in this respect. Narrative obviously more impactful than hard stat sheets.
