When buying a skateboard. it's the deck you should keep in mind, to best measure what you should be going for. 

If your shoes variate between the sizes 35-40 we recommend you to buy a skateboard with a deck between 7.25-7.5

If between 40-45 in shoe sizes we recommend getting a board between 7.5-7.8 since that will fit the sizes well.

If your have larger feet than the size 45 it is recommended to buy a board between the 7.8-9 inches that way you'll have better control of the board when doing tricks and riding on ramps.