Edit: I'm a dope and forgot about the share spells feature.
Edit II: I'm a dope and forgot that the share spells feature only works on "Target: You" spells.
While the synthesist and eidolon are fused, they are one creature. When you cast enlarge person on yourself, you're really casting it on your combined form (remember, you can't target either component creature separately, only the combined creature). However, enlarge person only works on humanoids, and you count as the worse of outsider and humanoid types whenever it matters. If you cast a size-changing spell with "Target: You", though, it would work, as then the share spells class feature kicks in, rendering the type distinction moot. (Expect some table variation as to whether targeting yourself is good enough to qualify for "share spells", but as written it doesn't work.)
Bull's strength works fine.
The rules regarding casting spells with a target of "you" on the eidolon and being able to target the eidolon with spells that do not normally affect his type are two separate statements.
- The summoner can cast spells with a target of "you" on his eidolon as a touch spell.
- The summoner can cast spells on his eidolon that do not normally affect the eidolon's type.
Good point. Note that there is still a FAQ pending (for the past six years....) on size-changing synthesists in general, so again, Expect Table Variation.™
