Patrx2's answer is spot on.
Playing a recorder in tune and on pitch requires a certain amount of breath pressure. If you don't put enough air into a recorder it will be flat and soft; too much air and you'll be louder and sharp. The fingering for each note also contributes -- the more fingers down the softer the note will be and the less pressure you need. The highest notes are the loudest by far.
Finally the voicing of the recorder, i.e, the combination of the design of the windway, hole placement and size, and bore characteristics determine how loud or soft it can be played and still be in tune.
Good plastics nowadays like Yamaha and Zen-on are fairly quiet, imho. Altos can easily keep up with acoustic guitars and harmonicas. Violins are a different story -- they have to be played with a little delicacy to not overpower a recorder. Because an alto is usually playing around C5-C6 (one/two octaves above middle C) they tend to be easily heard with/among singers.
It is true that the larger the recorder the quieter the sound. Tenors are much more mellow and the standard Yamaha plastic isn't that expensive, so may be a good choice for you.
There is also an easy trick to mute a recorder without interfering with the pitch: fold a tiny rectangle of paper into a V and carefully put it into the windway to cover a small part of the labium (the edge). Sort of like this: }- <- where } is your mouth, the first - is the mouthpiece, < is the piece of paper and - is the labium. I've used this trick to play in hotel rooms and have never had a complaint lodged against me.
Good luck with your journey. Recorders are wonderful instruments that are indeed easy to learn to play and are as rewarding as any other serious instrument.