Tire sizing & Stock tire/wheel specifications

Stock WR250R wheels/tires

Front
Wheel size: 1.60 x 21
Tire: 80/100-21 51P (3.00/3.25)
Bridgestone TW301 51P 698mm OD 83mm TW

Rear
Wheel size: 2.15 x 18
Tire: 120/80-18M/C 62P (4.00/4.10)
Bridgestone TW302 62P 654mm OD 119 TW

Load ratings of 51 and 62 correspond to 430 lbs and 584 respectively. Yamaha says 400 lbs max, if I recall correctly.
A "P" speed rating is 94 mph/150 kph - I guess Yamaha had our top speed dialed...

A little more info on tire sizes and decoding:

Permissible Rim Widths For Tire Width
(stock rims 1.60 on the front, and 2.15 on the rear)

1.60, 1.85	70
1.60, 1.85	80
1.85, 2.15	90
2.15, 2.50	100
2.15, 2.50, 2.75	110
2.15, 2.50, 2.75	120
2.50, 2.75, 3.00	130

Width
The first number in a tire size represents the nominal width. Width is measured in a straight line from the furthest point on one sidewall, across the tread, to the furthest point on the opposite sidewall.

Aspect Ratio
The second number represents the aspect ratio. Aspect Ratios indicate a tire's cross-sectional profile. The smaller the number, the lower the profile. It expresses the height to width ratio as a percent. A 90 aspect ratio means the tire's cross sectional height is 90% of it's width.