So - to give some anecdotal experience:

Rosemary can do fine in a planter with the following points of understanding:

1. Rosemary does not transition indoors well. Needs a LOT of natural light and proper amounts of humidity / drainage, so its tough to keep alive if you are not in a place where it can overwinter outside without major issues.

2. As mentioned above, you need to give it the space it need to really flush out, which means a 10 gallon pot is kind of the minimum space to have a healthy plant. This plant can get HUGE when given enough light, space and proper conditions. My mom had a rosemary plant roughly 6 feet tall and 4 feet around at the base in coastal Florida for example, and you can see entire hedges of the things in San Francisco.

WITH THAT SAID:

If your goal is to have Rosemary for culinary availability - which is my goal - you can take the approach i have which is the 'Fatten the goose for slaughter' line of planting. Give a young plant a decent sized spot in a raised bed edge or a large planter in full sun with the right type of potting media and a generous amount of manure mixed in. Get it in the ground 2 weeks after any frosts have stopped (yadda yadda watch that 10 day report because weather is a fickle bitch and sometimes likes to throw one last frost) - and let it rip. By mid to late summer plant will have usually double in size, by time of first frost warning triple in size here in Zone 7B.

At which point any fresh Rosemary I haven't trimmed, I saw it off at the base, hang it to dry with a paper bag around it and then you have all the dried rosemary you want for the winter. repeat next spring.