Hi & welcome,
'Boywounder'... do you wound boys, or is that the name of your horse, or...??? Just had to ask!
Agree with Beverley that it would be a very good move to get some lessons before buying, along with doing lots of research into keeping/training/caring for them, as I get the idea you're in the very early days of learning about horses(?) There's another thread I just replied to in the new to horses section, about a guy with a weanling & a 2yo, which will give you some tips on basic care & training you need to learn about.
But to answer your questions, size as in height, will depend on build as to what can carry you well. As a basic guideline(lets not get into nitty gritty if Cherie or bsms are reading...) I agree with the '20% max' rule - that is, the rider should not exceed 20% of the horse's weight. There are many exceptions & considerations, but that's a good starting place I reckon. So I'd be generally looking for a horse around 14.3-15hh for your size.
And what else should you be looking for in a horse? As you're a beginner, well trained & easygoing for starters. Probably no younger than 6-8yo, sound, healthy, well built. When you get to the stage of ready to buy, take a trainer or other experienced horse person with you. Even if you're experienced, another set of experienced eyes is a good move I reckon.