we know that
the ratio of the areas of similar triangles is equal to the squared scale factor
so
Area triangle ABC/Area triangle DEF=(scale factor)²
if area triangle ABC=area triangle DEF
then
(scale factor)²=1---------> scale factor=1
so
the length of the longest side of triangle ABC is equal to the the length of the longest side of triangle DEF
therefore
the answer is
the length of the longest side of triangle ABC is equal to 25 units