Most cartoon lettering is sized between 8 and 10 points; a good cartoon font should have excellent readability even at sizes as low as 7 points. Each artist must find the size that is large enough to maintain legibility, yet small enough so the words don't overpower the image. Increasing the size of your lettering decreases the number of characters that you can fit on each line of type.
Print some test copy at 7 points. Look at it carefully and consider the following:
Is the text readable at a glance?
Do the spaces inside characters such as e and o become clogged?
Do letter combinations such as ill melt together into one unit?
Are letters like m and n discernable from one another?
Do letters like L and I blend together to form other letters, like U?
Also ask a few friends to read the samples and watch them as they do. Do they squint when they read the type? How long does it take them to read it?
It really doesn't matter whether you letter your comic by hand, choose a computer font, or develop a digital font on your own. The only thing that matters is the legibility of the print. If the lettering is difficult to read, it's impossible to build the rhythm and pacing that a great gag or a dramatic story demands. In the eyes of the reader, you're over before you start.