You can easily-enough prevent tables from adjusting themselves by:
1. Unchecking the 'Automatically resize to fit contents' option (Table Tools>Layout>Properties>Table>Options);
2. Specifying the table width (Table Tools>Layout>Properties>Table>Size>Preferred width);
3. Specifying the column width (Table Tools>Layout>Properties>Column>Size>Preferred width); and


4. Specifying an exact row height (Table Tools>Layout>Properties>Row>Size>Specify height)

You cannot have the cursor automatically jump to the next cell when typing in a table. If you want to simulate an underlined, multi-line input box:
1. Create a single cell with the required overall diemsnions;
2. Set a right-aligned tab-stop on the cell's right margin (remove any other tabstops);
3. Press Ctrl-Tab a couple of times to insert tabs into the cell (each one should now occupy a line on its own);
4. Select the whole cell and apply underlining;
5. Adjust the paragraph font, before/after spacing and line spacing to match your desired input format;
6. Press Ctrl-Tab however many more times you need to finish filling the cell.

From now on, anything typed into the cell will be underlined and will wrap until the cell is filled, without expanding the cell - the excess tabs will simply be pushed out of view.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Thank you for the prompt advice. I'll let you know how it works in a little bit when I try it out. However, I have no idea what a right-aligned tab stop is, but it's something I can google if you don't check back before I get to work on it.