Most commercially available cores have published AL values ..
The formula for inductance is:
L = n² x AL
where L is the inductance in henries and n is the number of turns .. https://www.bytemark.com/products/ferral.htm
The current density of the winding can be for example 3 A/mm² ..
So, if you need 6A the cross section should be ≈ 2mm² ..