I'm with Verdun - your garden's about the same size as my back garden. I could accomodate a big greenhouse - but then what?
No point in having a huge greenhouse and raising all sorts of plants if you can't put them anywhere!
It's a balance - you usually need other uses from a garden.
I have one glass greenhouse that is attached to a wall. Next to it is one that you just put together and is four tiered, another one of those by the garage door. And then last year I purchased the long cold frame for all my seedlings for 2016.
We are in the middle of putting up a 20ft x 10 foot it's big and I'm was wondering the same as you. I'm sure I'll grow into it?