Absolutely. That would be a very good way to test out sizes. It sounds like you have the right idea.
Another popular techique is to use a "utility texture", a grid or checkerboard pattern that you can use to see stretching and make sure the texture looks clear before you paint the real thing. I usually keep several around in different sizes and colors.
This site has several good ones:
http://www.pixelcg.com/blog/?p=146
Here's a picture of somebody testing with one, so you can see how it works:
http://img.photobucket.com/albums/v6..._vtec/UVs1.jpg
And the UV map for the same model:
http://img.photobucket.com/albums/v6...c210_UVMAP.jpg
The important part is to notice how all the squares look clear and sharp, and for the most part they're all pointing in a sensible direction. The grid is helpful because any defects become super-obvious, so they can be fixed *before* you do the work of painting the texture. For example, you can see that the map area for the grill of the car needs flipped horizontally because the numbers are backwards. If there were any other problems--for example, if the texture were too small the numbers would blur--you can fix it right away before you've wasted time painting anything that's not going to look right when you're finished.
Whoever made this map did a very nice job. There's also a few other good details to notice on sizing. For example, the license plate takes up a lot more map space proportionally than, say, the hood of the car. Text tends to hold up poorly but solid colors hold up very well. By spending more of the texture on the license plate the artist was able to keep the license plate looking clear but still fit the whole thing on the same texture map.