Footprint of mine in the garage is about 6' x 4' and it's 6' high. I can do 11x14 in it, probably bigger if I got creative. The only thing wet in there is chemicals in trays; I do my rinses & selenium toning in the kitchen sink. Works for me...
Try and make it as big as your parents will allow. When it comes to darkrooms bigger is always better. Mine is 18'x12', 4x5 and 8x10 enlargers, 34"x8' sink, plus 12' of countertop. Wish I had another 50sq/ft. My first darkroom did not have a sink, just countertop for trays, basement had a utility sink I used for film and print washing. Size was about 8'x8' and did 11x14's wih 4x5 enlarger. I have also used walk-in closets when in an apartment, but ventilation was a big issue and very cramped and roommates didn't appreciate the smells.
You could get by with very little, it all depends on your needs.
My wetside is just a tiny kitchen table with a $25 utility sink. This takes up about half the table, and the remaining half can hold 4 8x10" trays just barely. Similary, my dry side is another kitchen table and my enlarger takes up half. I have no problems doing what I need to do at the moment. Just think about what you'll be doing and see if there's enough room to do that. Simple.
A tall wire bookshelf should accomodate all the odd's and ends you need.
One thing to think about - even if you have a big darkroom, it is really handy to have a nice, largish work area for work that you do in the light. It is also easy to make a work area like that multi-purpose, so other, non-photographer people in the household can use it.
A decent size work table, a good sink or two and counters and cabinets out in good light can be used and enjoyed by the whole family!
Think chemistry mixing, toning, trimming prints, matting, framing, negative sorting etc., while the rest of the family can think of things that they might like to do in a good, well lighted work area.
I'll add here that my advise is to get started with a temporary and limited setup first. Then, figure out for one's self that what scope of darkroom is best suited for his/her needs and how far he wants to go. One can easily get started in an average size bathroom and a closet. By the time one starts feeling the limitations of such setup, he will know what is required for the type of work he does in his darkroom.
I don't know if OT is talking about actually building a complete darkroom or have a space to setup one in more less a shared multi-purpose environment with his folks. But seems he is in a very fortunate situation. Here's what I do and works for me reasonably well.
In my spare bedroom, I have 3 folding tables each being 30 x 72 arranged in C configuration.
One has two enlargers
The other has misc items such as tools, extra lenses, a microscope, etc
The third has a large plastic tub with 4 trays in it and outside it, a deeper tray
The plastic tub I mention above was originally marketed for storing "stuff" under bed. It's a large and shallow rectangular tub. 4 trays are 8x10 sized. The last tray is deeper for wash water.
Film is loaded in this room and taken to a bathroom for processing
Print is made in this room and it gets placed in the deeper tub - which I take it to bathroom for a final wash
This room is about 12x12, I think. It's a small spare bedroom.
The only thing I wish I had is a plumbing in this room but I'm not willing to modify my house just yet this way. It's a bit inconvenient for this reason but it works well enough for me to enjoy the process.
Before having this semi-permanent setup, I did everything in master bathroom - dragging my enlarger in every time.
I have a very small space: 34" x 65" with a sloping roof. I print colour and B&W. Planning is key with tiny spaces. My darkroom is so small that I cannot move when I'm in there, I sit down and everything is within reach. One really useful thing is the Nova slot processor, I have two, and without them I would not be able to print as I simply don't have the space to lay out trays.
What's the largest space that you think you might be able to have?
Mine's 6'x8', and I can do up to 16x20 pretty easily. I tend to print smaller, though.
The sink is the smaller version of The Sink. They're pretty convenient and have a good back splash. Dimensions given are internal, so it could handle two 16x20 trays. I used standard shelf ware next to the enlarger, so they go all the way up and down the wall. There are also shelves above the cabinet on the lower left.
If it helps, I've attached a floor plan. Another interesting configuration is to swap the door and the film drying cabinet.
Water is the most important consideration. Managing water, especially if you don't have plumbing in the space, is more important than the size of the space.
Poorly managed water will kill your work.
Of course, if you have plumbing then start looking at size and arrangement.