If you select "Fixed volume", you can see it has a fixed value of 0.1 as a default. But you can actually put a MQL4 formula over there without problem. You can use MQL4 functions, such as AccountBalance() or AccountEquity() or others. Or variables.
For example:
AccountBalance() * MyVariableName
Value such as 5% will probably not work, better write it like 5/100