You must understand that this is not a critical condition for riding dirt bikes. These bikes are powerful, so you do not want to take the risk.  If you are at an initial stage, it should be better for you to ride the bike according to your height and size to have better control and stability. If your feet can be grounded, it will impart more confidence to you, and you will feel free to ride a bike.  But you can also ride a motorcycle even your feet are not grounder, and you have to be proficient in controlling the bike; otherwise, you can suffer an accident.