I'm learning Paul Gilbert's technical difficulties. It's no problem between fret 0-12. But when I try to reach frets 15-19 I'm having trouble with my finger's positioning. (Usually it's with the lower E and A string. D: ) I think it may be because I'm not that tall (158cm only) and my arm length is the one that complicates this. Are there any tricks wherein I can play. I tried to compensate for this by playing the guitar where it stand and not in the usual slanting position. Please help. Thanks a lot~
We really need to see what you're doing now before we can help; there is nowhere near enough information in this post.

For the record though: it has nothing to do with your height or arm length. That's just an excuse.
R.I.P. My Signature. Lost to us in the great Signature Massacre of 2014.

Quote by Master Foo
“A man who mistakes secrets for knowledge is like a man who, seeking light, hugs a candle so closely that he smothers it and burns his hand.”

Hahaha, yeah Zaph is right. They are excuses.
Make adjustments to your posture until it works. you can try classical position and/or holding the guitar a little further away from your body. Footstools can help.

Video/pics/better description.
I have three different "go to" postures that I use in my practice seat. 2 are leading forward, one with the guitar on my right leg, the other with the guitar on my left leg with my leg raised by a box or something. Then I sometimes play leaning back when neither of those are working particularly well. But I usually put on my strap if I want to sit down and lean back while playing.

The problem is I can only sweep pick well in one of these positions. So yes your posture can limit you but just find what is comfortable and don't forget to also practice standing up.
If you are playing standing up and have problems reaching those strings in said frets then i assume that your strap's too low. this is common nowadays.
Thank you guys for the feedback. I've been wondering for awhile if height or arm length really mattered and I've confirmed that it does not. It was just an excuse lol. Thanks for pointing that out. I think my posture is quite right. I searched for the different postures and found out that the guitar being around 45 degrees when I play it enables me to shred easily. Thanks a lot guys.

This is the exact same posture I'm using to shred right now. Lol, thanks. This is really helpful.