dismiss and reconjure as a medium creature after doing the necessary adjustments?
The Large evolution, if you spend 6 extra evolution points over the base of 4, gives you a Huge eidolon -- and you then do face the problem of the eidolon not fitting anywhere that a Huge creature won't fit, meaning that you would have to dismiss and later re-summon the eidolon in a cramped dungeon.
The Transmogrify spell is the only method that I know of other than the rebuild for leveling up to reduce a Huge Eidolon to Medium or smaller size -- and that can be expensive since Transmogrify costs 1000 GP each time you cast it. The Evolutionist archetype lets you cast this spell once per day for free.
There is Reduce Person, but that would only get you down to Large size from Huge.
This is part of why I didn't allow my party to take stuff like Elk sidekicks and stuff. It either is a pain in the ass (or big nerf) for the party, or it's a pain in the ass for the DM to create interesting dungeons (or magically all dungeons have 15 ft hallways for some reason.)
Don't get me started on Ladders, squeezing, and other things that allow interesting areas by comparison to if you have to carry a damn Elk around lol
Ah, I didn't know you were using mythic rules. Mythic Reduce Person should do the trick, if you only need to shrink your eidolon for a few minutes.
i afraid the only spell i know are mythic version of reduce person.This is part of why I didn't allow my party to take stuff like Elk sidekicks and stuff. It either is a pain in the ass (or big nerf) for the party, or it's a pain in the ass for the DM to create interesting dungeons (or magically all dungeons have 15 ft hallways for some reason.)
Don't get me started on Ladders, squeezing, and other things that allow interesting areas by comparison to if you have to carry a damn Elk around lol
Any spell of the polymorph school, starting with Alter Self.
Share spell allows you to target your eidolon will a wide range of polymorph spells that specify size while affected.
Or you could use Devolution
The main problem with Alter Self is that it does cost you any form based evolutions while it is in effect -- but if all you want to do is get a Huge eidolon through a small space, that would do the trick.
If you think you will have some time to spare after going through that narrow space, you could always dismiss your eidolon and then perform the ritual to bring it back once you are in clear terrain.
A large creature can move pretty much anywhere a medium creature can by squeezing.
Squeezing: In some cases, you may have to squeeze into or through an area that isn't as wide as the space you take up. You can squeeze through or into a space that is at least half as wide as your normal space. Each move into or through a narrow space counts as if it were 2 squares, and while squeezed in a narrow space, you take a –4 penalty on attack rolls and a –4 penalty to AC.
When a Large creature (which normally takes up 4 squares) squeezes into a space that's 1 square wide, the creature's miniature figure occupies 2 squares, centered on the line between the 2 squares. For a bigger creature, center the creature likewise in the area it squeezes into.
A creature can squeeze past a creature while moving but it can't end its movement in an occupied square.
To squeeze through or into a space less than half your space's width, you must use the Escape Artist skill. You can't attack while using Escape Artist to squeeze through or into a narrow space, you take a –4 penalty to AC, and you lose any Dexterity bonus to AC.
Yep, it is just a matter of time vs. resources. A squeezing creature travels at half speed and does not allow anyone to pass it, so you would want to consider alternatives if that slows the party down too much.