And, the only time where wheel size makes no difference is on Friction Drives (as Dave implies.) With friction drives, the ONLY two variables in determining ground speed are engine (drive spindle) RPM and drive spindle diameter.
In other cases, gear ratios are used to reduce engine RPM to a more manageable wheel RPM - usually in the 300-400 RPM range for a 26 inch diameter wheel.
For a given wheel RPM, a larger wheel diameter has a correspondingly larger wheel circumference, and thus, a correspondingly greater distance traveled for each wheel rotation (and, a correspondingly greater ground speed.) However, the torque is reduced by exactly the same ratio of wheel diameters, when increasing wheel diameter.
Think about it... If you're tightening a nut on a bolt, and the the bolt is spinning , you put a wrench on the head of the bolt to keep it from turning. If you use a long wrench, it's dead easy to hold that bolt, but if you do it with a stubby wrench or rachet, it could be really hard to keep the bolt from turning. The stubby wrench is equivalent to hte small diameter wheel - the distance moved (speed) is less, but the torque is greater. The long wrench has more distance (speed) moved but less torque.