You can start with a glass tank, or build your own acrylic tank as the base of your sump. It depends on how much room you have under your cabinet.
Generally, it is best to get as big a sump as you can fit. So basically start out by figuring the biggest tank that will fit in your space, remember to leave enough room so it will go through cabinet doors, & leave some room on top for maintenance (a foot is good). After you get that dimension, you can figure out the rest.
"The rest" depends on what you are keeping, SW
... etc. This is a good place to start:
Melevsreef.com - What is a Sump?
<Look at the rest of the site too ... there are lots of good info there.>