You need to figure out what you're willing to spend for peace of mind and go from there.
In my opinion the best upgrades are from Brembo, AP Racing, Alcon and Performance Friction but they'll come at a premium as good brakes do. Size will be determined by the manufacturers offerings. Generally they are matched to the size of your OEM master cylinder.
There are cheaper kits around but you might find that they aren't supported as well locally or are multi-piston bling. The availability and price of replacement parts might also be an issue.
I wouldn't buy any kit recommended from a forum vendor unless if you were familiar and confident with that product. The amount of crap that get peddled as quality these days is disappointing.