Calculating the maximum length of a fully matured mRNA?

As you should know, the mature mRNA will not have introns.
If you want to determine the full length of mature mRNA, then do the following:
1.) 5'G cap: tends to be about 5-7 G's.
2.) Exons have different lengths, depending on the gene. That information will need to be given.
3.) polyA tail: Length varies depending on location in genome and type of gene being transcribed.
They can vary from just a few A's, i.e. 20, to about 1000 A's.

So to determine the full length of the mRNA, you will need the above information first.
Or is you are given the pre-mature mRNA length, you can subtract out the introns. You generally can tell where an intron begins and ends by sequence alignment and/or specific sequences for splicing mechanism.

good luck.