A trundle bed earns its place in a small room only if it slides out, sits level, and leaves room to walk around it. Most of the trouble comes from buying the bed before measuring the floor it has to live on. Measure first, then compare.
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Measure the room the way the bed will use it
Take the width and length of the floor the bed will sit on. Then measure the space on the side where the trundle rolls out, because that is the part that gets blocked. Write down how far the drawer reaches when it is fully out. Add the path a kid needs to walk around it without stepping on a sleeping bag. Measure the doorway and the door swing too, since a bed that fits the room but not the hallway is the one that goes back.
Check the height and the mattress
Ask the seller for the height of the main bed and the trundle when both are assembled. Ask whether a mattress is included for the trundle or whether you buy it separately. A trundle mattress has to fit the frame exactly, so bring the frame's inside measurements with you if you are shopping for one.
Check the stops and the rails
Look for a locking or stop mechanism that keeps the trundle from rolling out on its own. Check that the bed has rails or a clear edge on the open side for a child. Ask who assembles it, whether it needs two people, and whether the instructions can be followed by one person on a Sunday afternoon. These are the points where a bed looks fine in the store and then turns into a project at home.
Decide who makes it up
A sleepover bed gets made up more often than you expect. Name who makes it, where the extra sheets live, and who folds it back on Monday. If the trundle is used on school nights, decide whether it stays out or slides under each morning. Write that rule down so a kid is not guessing at a sleepover on the way out the door.