A tesseract has three diagonals and you didn’t specify which diagonal. This is not a problem. I’m trained to read minds and find missing questions and missing parts of questions asked by the lower level primates, like you, that visit this forum. I can do this even if I’ve had too many banana daiquiris. Here’s a sample of my work.
http://web2.0calc.com/questions/hey-guys-i-posted-a-few-questions-and-i-can-t-find-them-anywhere-i-posted-it-yesterday-but-it-s-gone-what-do-i-do
Now, to your question.
I’ll give all three diagonals (each is a unique line connecting two vertices)
for an 8 unit tesseract.
\(\text {In 2-D “the face” (a square.)}\\ d = \sqrt {a^2 + a^2}\\ d = \sqrt {2a^2}\\ d = \sqrt {2}* a\\ d = \sqrt {2}*8\\ \)
\(\text {In 3-D a line connecting two vertices not on the same face (a cube). }\\ d = \sqrt {a^2 + a^2+a^2}\\ d = \sqrt {3a^2}\\ d = \sqrt {3}* a\\ d = \sqrt {3}* 8\\ \)
\(\text {In 4-D a line passing through the center of the “hyperspace” cube }\\ \text {and connecting two vertices (a quadragonal). }\\ d = \sqrt {a^2 + a^2+a^2+a^2}\\ d = \sqrt {4a^2}\\ d = 2a\\ d = 2*(8)\\ d=16 \\ \)
This is quite simple. Isn’t it? Math wizard skill isn’t needed for this. I know because I solved it and math wizard isn’t on my resume (yet). A genetically enhanced chimp taught me how to do this because I was fascinated by the art of hypercubes, and now I’m also fascinated by the math of art.
IF you want to test for math wizard skills, you will need to bump it up a notch or two.
GA
+2440 
so I’m not including language of that sort in your greeting.