Mag scales can be cut to length but glass scales can not be cut.
To measure your travel, move one axis to it's farthest end. Mark a line some how some where for reference, then move that axis to the other farthest end. Measure the distance traveled. The scale you want go choose should be 1/2"-1" longer than your travel or choose the next size up that is available. Generally you don't want to choose a scale that's too long as the actual size of the scale is longer than it's rated travel and the amount sticking out past the table may get in the way. If you choose one that is shorter than your travels you'll risk damaging the read head.
Scales are generally offered in metric sizes (imports). They're usually offered from 100mm - 1200mm in increments of 50mm but depending on the manufacturer they might have some in increments of 25mm & even longer than 1200mm.