How do I adjust the size of the data entry fields?

How do I adjust the size of the data entry fields? For example, if I want to enter a quantity of 25, or 250, I can only see one of the digits and it is quite unclear that I actually entered the two digit or 3 digit figure until I hit the add to cart button and it is only then I can see the confirmed quantity in the shopping cart.

Regards
David.