A typical swatch is 20 stitches by 40 rows. But the bigger swatch you can make the better.

Also, it is usually best to use the same stitch in your swatch that you are going to be using in the piece. Like use the particular lace stitch, color st, etc. Or if it is plain stockinette st or garter st, use that.

Sometimes people like to knit swatches just to see what different stitches look like with different types of yarn. If you are really creative, you might find that you like to knit quite a few different swatches with different size needles to find the exact texture you are looking for in your garment. Smaller needles will make a tighter knit.

After knitting your swatch, if you really want to be accurate, you should wash and block it, and then measure.

Also, when you are measuring, count your number of stitches across several inches, and then divide back by that number of stitches. You could very well end up with a decimal number, which will give you a more accurate gauge.

But if you are in a hurry, and like to take risks - just knit a small, quick swatch, and measure it.