Here's what I did to learn a lot about the size of (round brilliant) diamonds and my girlfriend's preferences. It also shows how silly and obsessive I've been about the whole process of buying an engagement diamond.
I was lucky enough to find an old used A.D. Leveridge millimeter gauge on eBay for $25.00. It works beautifully to measure the diameter of loose stones. Good luck finding a similar gauge for a similar price, but you never know.
I was also able to find loose CZ's on eBay for less than $2 in just about every size imaginable. I have 5mm, 5.5mm, 6mm, 6.5mm, 7mm, 7.5mm, 8mm and 8.5mm CZs. So basically for less than $40 (which is less than 1% of what I'll eventually spend on a diamond), I have learned:
1) How to use a Leveridge gauge to measure loose stones
2) How to get an excellent idea of the true size of diamonds I'll be looking at
3) CZ's are not marketed at the exact sizes that really are (my 6.5mm CZ turned out to be about 6.4mm)
4) My girlfriend thinks 6.4mm is too small, 7mm is too big and somewhere around 6.5mm to 6.6mm is just right
(Oh yeah, and I learned how to always tell a CZ from a real diamond)
All of this knowledge has freed me from the concept of thinking about diamonds in terms of carat size and now I think about them in terms of millimeters in diameter, a fact which will probably save me hundreds of dollars on my eventual purchase.
The practical example of this is simply: which looks bigger, a 6.5mm 1.00 carat diamond or a 6.4mm 1.04 diamond? Or how about a 6.48mm .98 carat diamond vs. a 6.45mm 1.00 carat diamond? Your wallet will feel the difference.