How do you find the magnitude of a force

F=ma
F=mg=w

Here ,
m=mass of the body
a=acceleration produced by the body
g=gravitational constant (value=9.8metre/sec.)
w=weight of the body

The magnitude of a force is the square root of sum of the squares of its orthogonal (Euclidean) components. In general, at a point a force will have vector components (x,y,z).
Hence √(x²+y²+z²) is the magnitude of the force.
When more than one force is exerted at a point the net force has magnitude and direction determined by the net components √{(Σx)²+(Σy)²+(Σz)²} ← note the placement of the radicals
Example: at point p, force A has components (1,1,3) ( in Newtons, say), force B has components (-1,0,2) (N)
THe magnitude of the net force is √{(1-1)²+(1+0)²+(3+2)²} =√(0 +1 +25) =√26

It depends what information you have.

For example, if you know the mass and acceleration of an object, you can use F=ma to calculate the resultant force, F.