The best way to make pleats all the same size is to measure the intervals. For example, if you want to make multiple one inch pleats, place a pin in the edge of the fabric two inches apart. To make the pleats, fold the fabric right sides together and match up two of the pins. Remove one of the pins and use it to pin the fabric together where they met. Then push the folded part over to whichever side you wish and press it in place. Repeat around to make as many pleats as you want.
I hope this answered your question.