Sizeing questions...

[epanzella]

I think the biggest inconvenience of a lathe too small is when turning long parts that won't go thru the spindle bore. You can work around other issues with speed & feed changes but introducing a steady rest into the mix is a time killer.

 

[DaveD]

I've been in this hobby about 5 years. Was a woodworker for at least 40 years prior.
I've found that mass and rigidity is important. Can't have too much of either.
Same for horsepower. I have a 2hp rf45 clone mill. It's 'adequate' for my needs. Weighs about 700#. No where enough to take 'heavy' cuts without chatter.
It's a dovetail column. Much better than a round column where you lose your positioning when you raise/lower the head.

My lathe is a 2500# 14x40 Takisawa with a D1-6 camlock spindle. Very nice for my novice abilities. It's ridgid and has a 5 hp, 3 phase motor. Don't let 3 phase bother you. I have a 10hp rotary phase converter setup I built. If you are handy it's not a problem to build one. Mine has a 220v/50a input. Need to be comfortable doing electric work though.

Cam lock spindles are nice. D1-4 is your typical smaller one. I love my D1-6.
eBay is a good place to find used tooling. Stay away from D1-5 camlock spindles. There is a lot less on eBay for D1-5 stuff.

Aloris type wedge lock tool posts are typically the tool post of choice once you get out of the lantern tool post style typically on the 'smaller' machines.

Lastly, expect to spend at least as much money for tooling as you do for the basic machines. That could be a subject unto itself.

Just realize, you can do small stuff in a big machine. Typically can't do the reverse.

 

[Silverbullet]

Most big jobs need bigger machines. I'd suggest if your lathe says 12 x. 36 then the work should be no bigger then 10" x 24" if it's bigger it will be extremely hard to do. At the chuck it may swing larger but I doubt it will over the cross slide. Mills are another item entirely Bridgeport can do amazing work on some items that are way to big for them. But don't try it if your not use to handling big heavy items. If your a hobby machinist it's just that for sizing too. Leave the big stuff to the shops set up for it. With big heavy parts things can go bad real quickly.
Most small machines are underpowered for many jobs . There just not made for real machine shop durability. So the old addage go big or go home works for machines.