Area of polygon of hyperbolic disc Let us consider hyperbolic disc. I use uniform tessellation {5,4}.Here 5 stands for pentagon, 4 for number of polygons sharing the same vertex. 
{hyperbolic disc}
There exists formula which defines area of convex hyperbolic polygon:
$$A_{m} = \{ \pi(m-2) - (a(1)+...+a(m))\} \frac{1}{-K}$$
where $a(i)$ is interior angle. K is Gaussian curvature, which I define:
$$K=-\frac{1}{l^2}$$ 
If I apply this formula for pentagon polygon I obtain that area of each polygon is given by: 
$$A_{5} = \frac{\pi}{2} l^2$$
So it follows that type of uniform tessellation together with Gaussian curvature define area associated with polygon. Is it correct?
Is it possible to have tessellations with {5,n} where n is not 4?

First I have to point you at a misconception you have, and I fear it will be confusing.
(I only understand just about half of it myself)
The disk model you use is the  Poincare disk model (see https://en.wikipedia.org/wiki/Poincar%C3%A9_disk_model ) and has a fixed absolute distance and a fixed curvature of -1 (this curvature is just build in in the model) 
off course you are free to use another distance scale with some formula $1a = i^2 c$  with $1a$ meaning the length of a segment of one absolute distance  and $i^2 $ your distance measurement of which $c$ is an unit length.
And this will result in a change of curvature. (but I do hear different opinions on what the curvature then or even what the unit of curvature is. so hopefully somebody else will help  you here.
In absolute measure the pentagon you describe has a fixed area of
$$A_{5} = \{ 3\pi - (5 \frac{\pi}{2}) \} = \frac{1}{2}\pi $$
And using the formulas at https://en.wikipedia.org/wiki/Hyperbolic_triangle#Trigonometry you can calculate the side lenghts
There is a  hyperbolic tesselation {5,n} for every $n > 3 $ 
see for example https://en.wikipedia.org/wiki/Order-5_pentagonal_tiling and 
https://en.wikipedia.org/wiki/Order-6_pentagonal_tiling
