Its a 15" 004 GIANT STP3 FRAME SET, i am 5'9'' and ride a 56cm road frame.

Without seeing you, or knowing your inseam/torso length. Or riding style. Or intended use for the frame, its hard to say. But myinstinct is that it is going to be too small. 5'9"/56 cm road frame says 17" mountain bike frame to me.

THe STP is a street, trail, and park bike which means you don't want the frame too big. IT really depends on preference. If you are going to be dirt jumping or riding street with it, i think that 15" would be perfect for you. But it really comes down to what you feel comfortable on.

For that type of bike, a 15" will be fine. Realize that frame isn't intended for all day XC rides. It's a more aggressive bump and jump type bike. It may take a little while to get used to a more cramped style, but this will be a benefit when jumping.

L8R

What you have to be careful of is this: It used to be common practice to make a trials/urban/hucking hardtail out of a smaller framed mountain bike. Now that there is a market for these bikes, the frames are being made specific to purpose. So you could well end up with the right size (that being a smaller than xc size) with the 17 " frame. Likewise, because they are making the frames smaller, the 15" frame might end up being way too small.

I am 'supposed' or 'fit' 17, 17.5 or 18. Doesn't matter the seat tube hight, am 5-9 also.
Its the extension of the body or the toptube. this can be correted with your stem.
(not by sliding the saddle back)

I 'chuck' a 16 seattube 20 inch toptube. You use the overpowering of the frame size diff (smaller) to your advantage. Some stuff is downright dangerous if the bikes to big, uncomfortable perhaps on a small frame- but very do-able.

On incline toptubes its the reach AND if you want to get tech- if you override size wise you lose the seat/bb inline pedal stroke. Does't matter so much if your freeriding/ jumping, but ride 20 miles and you want that seat up and correct leg/ bb action. F-your knees bad, seat to low and off the bb.

Sorry I'm not more helpful, should work as the type of frameworks is inclined.
Got short arms? That will help.

My 16 is like a bigger bmx, built it up that way.

I dunno?

Good luck with your decision,>Jef.

Well it all depends on your riding style. My P.3 frame is about 13" and it fits me great, although its pretty uncomfortable for xc. so if your into the street and park then that would be almost perfect, and since its a little larger and your a little shorter it should still do fine on the xc.