Mount sizes help

[Talas Leviann]

So I have a kobold summoner in my groups party. He is riding a huge skeletal dragon eidolon. However, someone told me that you can't ride a mount more than two sizes larger than you. I can't find any rules to support this. I have been allowing him to ride the eidolon but if someone could find the rules I could show him and we can decide from there to apply them or house rule it in his case. Thanks ahead of time!!

[Eltacolibre]

The only issue you will run into, you need a move action instead of a free action to mount or dismount when the mount is too large.

It's basically no different than a human riding an elephant for example.

[Claxon]

However, keep in mind that riding an Eidolon requires the mount evolution. I'm also not sure how he made it a skeletal dragon, unless that is just fluff.

[Talas Leviann]

He had the mount evolution. He also made it undead with some fluff