A quadrilateral can have an inscribed circle if and only if AD + BC = AB+CD. Sum of opposite sides are equal.
I made a GSP sketch of this. I uploaded a pdf file. If you want to find the length of the side, and if it's a general quadrilateral, then I don't think you have enough information. I'd say you need to know the radius of the circle and something about the central angles (between the radii that lead to the vertices of the quadrilateral).
If I know it's a square, then I can figure out the side length in terms of the radius.
Maybe I'm missing something.
It's true what coolge says, but I thought the problem was the find the lengths of all four sides in terms of the radius.