measure it in inches within 1/16th inch, and we can convert to mm. the front fork is probably easiest.. measure from the centerline of where the axle used to be to the middle of the brake pad when the brakes are held about 1/2way to the bars (about where they would have been touching the rims. get that within 1/16th inch, and multiply by 25.4 to get the radius in millimeters. multiply that by 2 to get the diameter and you should get the approximate wheel size. 700c is 622mm. 27" is 630mm. 26" is 559mm. on a real wheel, this is measured from the seat of the bead, which is about 1/4" from the edge of the rim, and just about where the middle of the brake pads should be, assuming the brakes were reasonably well adjusted.