So much depends on earlobe size, personal taste and cultural preferences.
I have attached earlobes and so huge diamonds would look ridiculous on me. I own two pairs of diamond studs, 1.62 ttcw in 18k yellow gold from Blue Nile and 2.13 ttcw in platinum from Whiteflash. I wear the 1.62 ttcw the most as they feel like the perfect size for everyday wear and my lifestyle as a SAHM.
I can’t see myself upgrading to bigger any time in the near future, I would rather invest the money into other pieces to add to my fine jewellery collection. I may reset the larger pair in yellow gold though as I would wear them more often (yellow gold is my preference) and it would make sense to have the larger studs in my favourite metal setting. I already had the yellow gold diamond studs when I bought the platinum ones.
I also live in the UK where larger diamonds are extremely uncommon and women don’t really wear much fine jewellery in general (obviously not talking about very wealthy enclaves in cities like London, Russian oligarchs etc lol).
Objectively, I don’t think you can go wrong with around 2-2.50 ttcw. They are big enough to look great/have presence but not be too big for most occasions, formal or casual. Bear in mind also that, depending on the size of your earlobes, you may be able to get away with much smaller than this to achieve the same look and save a ton of money - it really does depend on how the stones will look in your ears. Why don’t you buy some cheapo CZ studs in different sizes and try them on to get a feel for what size you feel most drawn to?