I published my solution via IWP and the fields where users enter data are adjustable. For some reason, there are some diagonal lines in the bottom right corner of every field and if I click on it, I can drag the field to expand a little. The data can still be entered, but it doesn't look very clean. Any thoughts on how to prevent this?

I noticed the little "expansion" triangles too. I think this came out with Safari 3 - have you tried other browsers?

I found it a most desirable feature as web rendering meant I originally had to design layouts with fields a little oversize to accommodate descenders in text.

For reference I found that a 12 point text field required 18 pixels depth to look good. I now use the 2:3 ratio throughout and looks fine for FMP and IWP. If the field is, say, 4 lines deep then it would (3*12)+18 = 54 pixels deep as you only need the space in the last line for the descenders.