I have to decide on which size to get 18" or 20" for desert riding could anyone help me out on which size is too big or too small. Or if it doesn't matter what size to get. Will it affect my riding? Thanx for any replies.

 

For desert riding, I would go with at least 20 inch. This will give you more top speed and ground clearance.
Most 18inchers are used on MX tracks only, for everything else people use 20 sometimes even 22 if your gearing can pull it.