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+1 on Dylandog's advice.
On a tight budget, aim for narrow and long, instead of broad and short. 40 X 150 mm (1"3/4 X 6") is a good size. You need a bit of distance to perform a good honing stroke....

Leon,

I really advice you to buy a yellow coticule, for sharpening razors. The blue is a bit too coarse to leave the fine edge most of us prefer. Furthermore it is very slow, completely...

Coticules are great. For centuries they have been THE stone for honing razors, and the yellow one produces a very comfortable shaving edge. They are fun to use, give a lot of "feedback", which means...