How to determine my bike's seat post clamp size?

My mountain bike came with a quick release clamp that reads ’34’ on its side (embossed or debossed, am forgetting. Reading online there are about 3 sizes for seat post clamps I went with i believe ’32mm’ something (lower than 34, since the next higher size ‘sounded’ too big to me), but it turned out noticeably larger, so i had to return it. So what does the ’34’ stand for? And how can I determine size without going to a bike shop?

-thanks