I would keep the hot water line at 3/4 inch. When you pipe the hot water to the far right tub I would add a 3/8 copper line and return it back to the bottom of the water heater. This is called a HYDRAULIC LOOP. If it is all on the same floor you might need a circulating pump to pump it back to the bottom of the water heater. Remove the crappy plastic hose bib and install a nipple, a tee, a check valve, a union and a valve. It could be 3/4 inch too. This line should be metal and insulated as well. You can re install the crappy plastic hose bib if you want but I install a full port 3/4 inch ball valve with a hose thread adapter on it. You will get hot water very quickly doing this. If the water heater is in a basement then the natural flow of hot water rising will force cooler water down to the water heater without a circulating pump.
If you are using a brand new water heater that you purchased from a store like Home Depot, return it and get one from a good plumbing supply house. I purchased a GE brand from HD, it did not last 6 months. Two holes formed in the tank. COMPLETE CRAP!!