I have an RF-31 mill/drill and a 4" Kurt vise fits it perfectly and allows full Y-travel. A larger vise would restrict Y-axis travel or put the rear jaw in a less than ideal position. My table is 8 X 28, so for your 20 X 7 table I should think a 3" vise would be better.
The vise size is not just about capacity. It must be bolted down to the table and this puts the rear fixed jaw in a certain position. That position greatly influences how much Y-axis travel you have. The bulk of the vise that projects behind the rear jaw can also contact the mill if the vise is large, further restricting Y-axis travel. Accordingly, you want a vise that is sized to the mill and table size.
Since 90% of your work will probably be done with the vise, having an accurate vise is really important. If I were you, I would buy a 2-3" screwless import vise that is accurate into the low tenths. This will give you accuracy, enough capacity that should match what your mill can handle and allow you time to learn to use the machine. During this time I would look for a used Kurt D30 3" Anglelok vise on ebay. They come up from time to time and generally sell in the mid to high $300.00 range - if you find a good one, buy it if you plan to stay with your mill. If you sell your mill you can sell the Kurt D30 for what you paid and move on ... and it will sell, I assure you.
A milling vise has to be pretty accurate and it must hold the work solidly without causing it to rise when you clamp the work down. A screwless vise will do all of this, as will a good Kurt vise. Size it to your mill so you can use all the travel the mill has and you'll be fine.