Exactly how bike wheels are measured is slightly complicated, basically you want one which is about 63-64cm total outer rim diameter. 26" wheels can also be used but are less ideal.
The wheel should:
~ Not be quick release
~ Have a normal thick axle (about 9mm diameter)
~ Have 36 spokes
~ Run reasonably smoothly
~ Have enough axle showing to attach to your pole mount, at least 4cm.