I am wanting to by a new mountain bike frame and I'm a bit confused by info I have read online and my own personal experiences.
I am 1.66cm (5' 4.5") and currently have a 16" frame. I use it mostly for road riding and getting to work etc but do use it for off-road mountain biking fairly often. I have the seat raised quite a lot and although I do lower for parts of off-road stuff, I never put it all the way down.
So my confusion is this. I have seen a second hand frame which I am very keen on getting and from my own experiences with my current frame I assumed an 18" would be what I would need. However, everything I read online suggests my current frame is the one I need.
In case anyone is wondering I got my current frame (bike) when I was about 13 (I'm now 24) and clearly haven't grown much as I am short

Help and advice appreciated.
Thanks