I got this in my email this morning. I don't see why if you used bigger pots why it wouldn't be useful for plants as well.
HANDY SEED SPACERS
by Jody Gilbert
When the time comes to plant a row of seeds, most gardeners
need to rely on some tool to measure the distance from seed to
seed. Of course, the basic approach is to use a ruler or
yardstick, but we recently came across an ingenious alternative.
You can use a flowerpot--2-inch, 4-inch, or whatever the seed
packet recommends for spacing--to mark the appropriate
intervals where the seeds should be planted. Just turn the pot
upside down and "stamp" it end-to-end down the row, making
sure the circles made by the rims are barely touching. Then,
plant the seeds where the circles meet. Since the pot circles are
uniform, this ensures that your seed spacing is consistent and
appropriate for the seeds you're planting.
Or how about this? My little finger is 2 inches long. My hand from side to side inculding my thumb is 4 inches. From my wrist bones to end of index finger 6 inches, and my hand spread out as far as possible from my little finger to the end of my thumb about 8 inches. I can combine any of these and make about any measurement I want.