It sounds like a problem a lot of people at my place of work get themselves into.
As far as I'm aware you can't increase the size in the drop down list box.
The problem usually stems from normal text/data being of a font size over 10pts, this forces the user to decrease the zoom that is used, if it's 75% or below I'll be tiny. but! you think! all the info on the sheet will be hugh if I change that.
What should be done is that the sheets/books should be set up at 100% zoom and so all the data is legable an easy to read. What a lot of people do is inrease the size of the font to make them legable and thus it creates the problem you are describing AND make it a bit of pain for setting up the page for printing.
Of course, this doesn't help if you've a lot of data and a low resolution, you just can't get all the info you want on the same screen.
The two ways I'd combat that problem is
A: increase the resloution on the users PC's (if it can be done), that'll give the users a bigger sheet area to view and you can set the form up at 10pts for normal text and the zoom to 100%
I'd use at least 800 x 600
preferably 1024 x 768
B: if it's not possible to increase the resolution because of hardware limitations, I'd suggest decreasing the font size to about 8pts and decrease the size of the columns/ rows to fit, then increase the zoom to a comfortable reading level and your drop down boxes should be bigger than the text it's self.
Thanks, I appreciate the work around. It worked perfectly.