I too added space to the comments field by altering the tables to make them 'mediumtext' instead of 'tinytext'. The tables I updated were:
0_comments
0_purch_orders
0_sales_orders
0_budget_trans
0_gl_trans
0_supp_invoice_items
0_trans_tax_details
0_voided
I also enlarged a few other comment type fields too:
description in 0_debtor_trans_details
description in 0_grn_items
Basically, you can do a search of company SQL that comes with the package and look for tinytext, comments, memo and description.
To enlarge the comments section of things like invoice entry, you might need more room on the screen too:
To do this, update eg. ./sales/includes/ui/sales_order_ui.inc
and change the line that refers to the comments which has
textarea_row(_("Comments:"), "Comments", $order->Comments, 31,5);
to:
textarea_row(_("Comments:"), "Comments", $order->Comments, 61, 20);
You might want to do this for other entry screens eg. for po_ui.inc etc. A global search on the text '->Comments' should get you started on which files you could modify.
As tclim said, you can also modify your report files eg. Invoice rep107.php. You need to be pretty careful doing this that you don't muck up alignments etc.
Lastly, don't forget you can get more text on your reports if you change the report fonts. I use a font called 'tuffy' (http://www.fontspace.com/thatcher-ulrich/tuffy) which fits alot on the page (it's narrow) and is quite modern looking. It's a technical job to add this font or any other to FA, but I have documented it in the FA wiki for those with enough technical know-how to use command line utilities and code PHP. You use an external utility ttf2pt1 which converts ttf files into font file the TCPDF framework can use. TCPDF is the framework FA uses for handling PDF files.
Pete