The wheel diameter should be 26".
My guess would be that you're probably going somewhere between 20 and 25 km/h (12.4-15.5mph) if you're traveling at a moderate speed on a bike like that. You can estimate the speed you're going by the gears you're in, and how fast you are pedalling. First, you need to count how many teeth the cogs on the rear wheel have. Then you need to count how many teeth the chainrings on the front have. Last, determine what your cadence is (cadence is how fast you are turning the pedals/cranks in rotations per minute).
Once you have those numbers, you can use this formula to determine roughly how fast you're going:
Assume you're in a cog with G teeth and chainring with C teeth, pedalling at RPM rpms.
Speed = (C/G) * (26" * 0.0254m/inch) * (120*pi/2000) * RPM = C*RPM*0.1245/G
This will give you your speed in km/h (I'm in Canada).
For example, if I'm pedalling 95rpm in a 32tooth front / 12tooth rear combination, my speed is 32*95*0.1245/12 = 30.6km/h = 19mph.
Hope that helps. Good luck with the wheel.