First you will need to set the paper size:
(2003)
File > Page Setup > Paper (tab) > Paper size (drop-down) > Custom > Select sizes for Width and Height equivalent to your cheques > Change any other relevent settings.

Then you will need to set up the Template:
(I would suggest you photocopy a handful of the cheques for this bit, as you will need to make adjustments!)

Create the cells that you need to fill in on the page (not sure if you are going to use this as a mail merge, but it would be the easiest method).
Try out by printing on the photocopies, and alter as needed, until the design is just right.

If you save this as a template, then it can be reused.

Hope that helps!