Let l be the prime length of one isoceles side.
Let s be the length of the third side.
Then P = 2l + s, where P is the perimeter.
Using prime values,
l = 2, 3, 5, 7, 11, 13, .....
2l = 4, 6, 10, 14, 22, 26, .....
Using s = P - 2l, = 25 - 2l,
s = 21, 19, 15, 11, 3
Possible lengths for the third side are: 3, 11, 15, 19, 21