I agree there should be some criteris for defining the "size" of a mod, but have no idea what that might be.
I know one thing that always bothered me was that Star X Vampires was in the "big" category when personally, I don't think it has any quests. There are only 2 "quests" 1st one has a courier telling you to go to a location, you go there and join the organization, and the 2nd quest is the exact same thing. Past that there are no other 'quests'
My take on sizes:
Spoiler Huge=
new world space with at least one major town, and a main quest line with at least 5 moderate sized quests
or
A wide reaching series of quests with about 40-50+ quests, multiple guilds, and/or branching storylines also factor in.
Big=
A new guild with 10-20 quests
or
A new world space with a bunch of misc quests
Medium=
quest line mods that don't fit in big, incomplete guild mods, mods with 3-10 small quests, etc.
Small=
Small additions to vanilla guilds/quests, short quests with only 1-2 quests, etc.
One thing that is an issue is individual quest sizes. one mod might have 20 throw-away "go deliver this quests" and be considered "big" while another quest mod might have a huge in depth quest line that is handled by a single quest. So mods kind of have to be dissected to see just how much questing a mod really has.
I'd say:
Spoiler Huge:
Integration: The Stranded Light
Kragenier's Death quest
Nehrim
Windfall
Big:
Heart of the Dead
Servant of the Dawn
The blackwood company
Lost Spires
Verona House Bloodline
Simyaz's 3 mods? (might be medium, or big)
Medium:
alyeid steps
ivellon
both GtA's
Origion of mages guild
daggerfall memories
sentient weapons
thievery in the imperial city
imperial legion faction mod
OOO
Small:
Star X
Knights of the white stallion
thieves arsenal
enhanced quest roleplaying
This is just a rearrange of the "big" category, the other categories look fine. The mods in "big" I didn't mention, I'm not familiar with. The sizes I chose are based on actual quest content... the old list based it on overall content. As an example of this Star X re-did all the vampire caves and altered the vamperism system thus under the old system it was a "big quest."