Is a 30x30 castle big enough? ( minecraft survival)?

No, lol
JK it is big enough
it should be 40 blocks high

Build it as big as you want, I would build it 100x100 depending on what you add into it, if you have lots of stuff you are planning to build in it, you can always knock a wall down and expand.

Without mods 30x30 should be more than enough to house a bedroom, workshop, storage room, and library (for enchanting). If you want to have an farm and/or an animal farm "inside" you'll probably want to attach a open air courtyard. You'll have to put any mobs farms somewhere else though.

As for height, I like to build my ceilings high enough that I can jump and not hit my head, about five~six blocks or so per level.