Typically the size of a control is changed by setting its Width and Height properties, but as the RadUpload has its dimensions limited by the MaxHeight, MinHeight and MinWidth properties, you have to modify them too in order to change the size of the controls. Here are the default values for these three properties:
MaxHeight - 300
MinHeight - 175
MinWidth - 350
Of course, if the size you need is in this range you don't have to set these properties explicitly.