"room to move" is open to interpretation. What if you are halfling or dwarf in the same area that is too small for a human?
A halfling is a great example here. Small creatures would most likely have the necessary room to use Evasion. Dwarves aren't small though, and supposedly they'd even have a harder time squeezing through certain places than humans do because of their bulk. For ease of gameplay though it's probably best to treat all same-sized creatures the same, although those with Powerful Build might have issues.
Having enough room to evade reminds me of a show called Fullmetal Alchemist. In one scene the main protagonist, Ed, is running from a boulder trap Indiana Jones style. He manages to evade it by squeezing into the corner between the floor and the wall. In his case he was small enough to do that where most other medium creatures would not have been able to.
That case is probably represented best in D&D by a circumstance bonus. Still, it brings up part of the issue at hand, which is possible lack of room and/or mobility (not the feat Dandu.
) A character not only requires the ability to move from a certain area, but also into another one.
In short, it's pretty much a judgment call for the DM on whether the character(s) have enough room to use Evasion or other abilities that require enough room to move. The fact that it's mostly a DM's call is given further credit due to the "room to move" rules being in the DMG and not the PHB.
