Hi,

this can have many different reasons, beside its size. Did you check successful transfection and mRNA expression of your transgene first?

Also be aware that western blotting of large proteins can be rather tricky, use thin gels and long transfer times (overnight) to achieve good results.

Best

Chris

Yeah siding with Chistian here.  I would say it's even unlikely that your protein's size by itself is the problem.  People clearly express proteins that size all the time.  You'll need to go through the full complement of troubleshooting.  Again, second Christian, mRNA is a good place to start.  If you can tag it with GFP, vs. GFP alone in the same vector, that would help too.