I have a 15KBTU mini split unit and the data calls out for 14/3 wire to run 220V, 15 Amps to the condenser. This seems kind of light to me but this is what is called for. My run is going to be around 90'-100', should I use heavier wire, say 12/3?
Thanks.

It won't hurt to move up to 12/3 or even 10/3 for future equipment. They don't normally expect a 100 ft run, too much line drop.

Most 15,000 btu/hr, 230 volt mini split condensing units only use about 8 amps. 14/3 is normally rated at 15 amps so 14/3 should work fine but it never hurts to go bigger.

I'm going to go with 12/3 and a 20 amp breaker. Do I need to put a disconnect switch next to the outdoor unit or am I okay with just the breaker inside the house?
Thanks.

That would be the safe way to do the installation. You could just install a sub panel. Is this a garage, barn, work area being connected to a home?

Electrical code requires a disconnect means within sight and readily accessible. NFPA 440-14

This is for a bonus room in my attic. I installed a sub panel in the room and I'm going to wire the mini-split off it. I think I'd just go with a disconnect switch/box and nothing more than that. I don't think I need another breaker or anything like that within sight and readily accessible.
Thanks.

That's your option but, if you ever decide to sell your home and need a electrical inspection done, it will be a electrical code violation. It also helps the service technician when servicing the unit and provides a way to shut the unit off in case of an emergency if no one is at home.

A circuit breaker in the panel is required, and a switch at the outdoor unit will suffice.