I don't know if this is still an active request, but in the spirit of participation, here are my two cents.
About how big do they get?Males - 150-180 bls
Females - 130-150lbs
They look like they'd be a little strong (saw one pulling our neighbor down the road) Are they easy to leash trainAs with any dog, the more time you invest in training, the greater the outcome. You really do get out of these dogs what you put into them.
I was told they make a good watch dog (not guard dog) and will alert you to danger, is this true?Turkish is a good watch dog and is the first to hear anything - unless we're being attacked by a fleet of vaccuum cleaners, in which case he will be hiding over in the corner.
Is it true they are one of the most gentle dogs?I think they are gentle but not entirely tied to their breed. Imagine the type of people that voluntarily welcome a 200lb dog into their homes? My experience has been that folks that make this kind of committment are gentle themselves - dogs are sponges for learned behaviors. If the owners are mellow, the dogs tend to be as well.
What health issues does the breed have as a whole?Bloat and stomach torsion is the highest among Danes. There isn't a specific cause and many schools of thought on preventative measures for this - one of which has become increasely popular and that is stomach tacking.
Also what is the best living environment for them (house, apartment?)Space - Space and More Space - Imagine how you would fill if you lived in a house where everytime you turned around you'd smack your nose on a wall? A big yard helps too, unless you don't mind picking up Man-Sized poop everytime he goes out.

A bit late but hope that helps any readers out there?