As measured (using fine grain sugar as test material) it has a 1/4 cup capacity. Though you would never want to put that amount of loose tea in it as the leaves need room to expand and water must be able to flow through them.
This infuser is less than two inches wide, however I'm not sure this is a good choice for portable tea-drinking. Loose leaf tea doesn't come out of this infuser as easily as it comes out of a tea ball. To clear the tea leaves out after brewing you will probably need water from a faucet, so you'll need to be around a sink with a garbage disposal.
Perfect for making cold brew coffee each morning. All you do is use the infuser and fill it half way with course ground coffee and steep over night at room temp for 12 hours. I like to let it steep for about 4 hours and then I place it in the Frig. Take the infuser out the next day and drink. I do not disturb the bottom of the mug. Try it you will like it!
That is a hard question to answer because it really depends on the tea. It will make a nice pot of tea or you can put less in the chamber for just a cup. If it is a standard loose leaf tea that is not too bulky about 3 tablespoons or so but some of the flavors that I enjoy have some bulk to them and honestly I don't measure I just fill it. I hope that this helped. Sorry I could not be exact.