The eidolon must be at least the same size as the synthesist.

[Jose Suarez 310]

Does it means that I can't take the large evolution/ hughe evolutions since I have to be at least the same size?

[Rynjin]

At LEAST the same size.

You can't be a Medium Synthesist with a Small Eidolon suit.

[Master of Shadows]

Master of Shadows wrote:

At least literally means: At the smallest.

pretty clear to native english speakers, but cleaner language might have been:
"The Eidolon must be no smaller than the synthesist."

[Jose Suarez 310]

So by all means the eidolon can be bigger?!?!?!?!?! can I have a medium character with a hughe eidolon?

[Master of Shadows]

Yes, but your GM will probably set all of his adventures in cramped dungeons with 5x5 corridors. Its the easy foil to oversized eidolons.

I would not suggest making a PC eidolon bigger than large, and then use Greater Evolution Surge to go huge as circumstances allow.

I did make an NPC Synthesist who dual wielded Huge Bastard Swords though. Perks of being the GM.

[Jose Suarez 310]

Thanks guys, I really appreciate your help master of shadows!

[Master of Shadows]

Anytime! I happen to be doing alot of research on the Synthesist myself lately so I had a good Idea of where to find the info.

Also:

This is a great resource that you may find helpful

[Rikkan]

Master of Shadows wrote:

I would not suggest making a PC eidolon bigger than large, and then use Greater Evolution Surge to go huge as circumstances allow.

You can't do that.

evolution surge wrote:

This spell cannot be used to grant an upgrade to an evolution that the eidolon already possesses.

And since becoming huge is an upgrade to the large evolution, you can't grant huge with evolution surge.

[Master of Shadows]

Ah, I missed that Line, thanks for the correction

[Lord Vukodlak]

Master of Shadows wrote:

Ah, I missed that Line, thanks for the correction

Ah but due to share spells a summoner can cast a prepared "reduce person" on his Eidolon despite it being an outsider. And the bonuses for a huge Eidolon far outweigh the penalties for reduce person.