We use a completely difference sizing system for water than drainage (f.U.'s).
You can click my link and look up the three relevant charts under factor values.
First, we add up all the factor values for each fixtures, then we use a second chart for demand factors, for either residential, multi residentiall, commercial, industrial...etc. and finally a size chart for the main.
(these all become second nature before very long...so it's not often we reference the book)
A 1/2" main will hold up to a factor value of 4.
On fixtures:
3/8" line is a factor of 1 (lav feeds & toilet)
1/2" is a factor of 2 (shower valve & KS faucet)
3/4" is 6 (Roman tub fillers, Urinal flushometers)
and 1" is 12. (not many fixtures have a greater than 1" feed...closet flushometer is about it.)
We then look up the demand factor for whatever the building is, a standard residence is .5 - in other words you multiply the total added factor values by 1/2 (.5)
A standard 3 piece bath is a total factor of 7 with hot & cold included...then you multiply that by .5 and you come up with 3.5, just enough for a 1/2" line.
We use 1/2" tee's in this situation, in fact most baths are fed with 1/2" line unless they're on the way to other fixtures and the demand calls for 3/4".