If you've got a couch, the size of your house doesn't matter, because that's where they spend most of their time!
For some reason, little ladies and danes go hand in hand. I'm a little "fluffier" than I used to be, but I'm still very short and look funny with a big dog. Imagine when I was 105 pounds and walked into Petsmart with my 185 pound male and 130 pound female. I got lots of odd looks.
For size - it varies on the genetics, but males are typically 32-39-ish inches and anywhere from 120-200 pounds. Females anywhere from 30-36 inches and 100-160 pounds. But it varies greatly from line to line.
As far as showing, the harlequins are the stiffest competition, so I'd avoid getting a harle unless you just REALLY like a challenge. A show marked harle is also much more expensive and will require being on a waiting list for quite some time.
For someone new in the ring with danes, I'd recommend a fawn. If you are cropping the ears, that gives you a little more chance for success, but many many judges are now putting up natural earred danes also. I can send you a list of judges who have put up natural earred danes if you ever need it.
A well bred show quality dane is generally in the $1,200- $2,000 dollar range. Pricey? Yes..but well worth the money for the quality of the dog. You are paying for the health tests and extensive care that has been put into these dogs as well as the time and labor that the reputable breeder has invested into assuring the dogs are healthy and of good quality.
If you decide on a color you're interested in, and let me know where you are located, I can try to point you in the direction of a reputable breeder.
In the meanwhile, if you would like to learn more about danes, feel free to click on the link in my siggie. And feel free to ask more questions.
Shall I show you some dane pics to entice you more?