the length is normally shaft diam times four ie 100mm but outside diameters vary but approx 1/4 wall
watch out some have a [--word removed--] size ie metric shaft imperial od
measure twice cut once still applies
I agree with Scottie. The likely size of your 25mm bearing is 100mm long. If you look at the Countrose site then you will find prices for Metric sizes and all the 25mm bearings are 100mm long.