Combo Box
 

I want to create a combo box that has different paper sizes i.e. A4,A5, DL
size etc. and when someone selects say A4 size in the combo box it then puts
it data 297
in a cell D1 and 210 in E1. I don't want there to be formulas in those two
cells, because I want people to be able to input custom sizes. Hence in the
combo box I want a 'custom size' option that deletes data in in cells DL and
E1 allowing them to enter their size of choice manually.
So I am assuming I'll need to do this with code?