Small is about 1/8 inch or less.
Alex caulk (siliconized acrylic)
A wet finger.
And push the caulk as you apply it.
Pulling does not get enough into the joint far enough.

brickeye...or others...

What do you do when you're working with stained wood instead of painted wood? It's not like you can stain the caulk???

Go to the hardware store and get painters caulk. You paint the caulk the color of the ceiling. You can caulk 1/8" or more but you should try to keep the gaps to a minimum by scribing the molding to the ceiling. If you're putting up crown molding you need enough meat on the top part of the molding so you can scribe.

"What do you do when you're working with stained wood instead of painted wood? It's not like you can stain the caulk???"

One of the reasons stained woodwork costs more is the material is better than 'paint grade' and it takes more labor to install without gaps, nail heads showing(they need to be filled with matching putty), and joints not closing perfectly.

The basic answer is no gaps.
For minor gaps ypu can mix tinted hardwood floor filler and use it to fill minor defects.
You can also purchase Mohawk filler sticks (tinted wax).
They have enough colors you can usually get a decent match.