For me personally I don't want to go over 2.5 per ear (or 5ctw for the pair) on diamonds. On colored stones I will go a little larger, but not too large because they don't sit properly. I use the large plastic backs on larger studs to keep them straight. Currently my largest pair of diamond studs are 1.60 ctw, and I find that for everyday wear they are perfect. I have a larger set of white zircon studs (NOT cubic zirconia but white zircon) that are equivalent to 4ctw and that I wear when I travel.
Some of you might find this interesting, my brush with huge diamond studs. My husband and I own a retail business and one of our customers fell into some financial difficulty. The wife decided to sell some of her jewelry to help out, and that resulted in the acquisition of a beautiful Tanzanite ring and a diamond and sapphire slide which I ADORE. Her mother also had a LOT of jewelry and wanted to sell some of hers to help out further. She said she had a set of diamond studs that were 5ct and all I could think was finally I would get the diamond studs of my dreams at 2.5ct each! That's not what she meant. When she came in with them, imagine my shock to find that they were 5cts EACH. Honestly, I didn't even want them. There is no way they would have looked good on my ears. (Plus she wanted more than either I or hubby was willing to invest. I thought she was asking a fair price, just not for me!) She ended up selling them to a jewelry store in town for what I thought was a good price, but it was quite a let down for me.