Cant find an answer for this, but a medium Synthesist Summoner is restricted to a medium eidelon wrapped around him?
Can I permanently enlarge the summoner then take Large ability?
The Fused Eidolon of the Synthesist needs to be at least the same size as its Summoner. So you could not make a medium Summoner and wear a small Eidolon. You can, however, use the size increasing evolution to make the Fused Eidolon bigger than its Summoner.
Yep, no need to enlarge the summoner first, you can technically play a gnome and put him into a huge eidolon, no problem.