In prokaryotic cells, the length of the mature mRNA for the introduced and expressed full-length gene will generally be the same as the length of the coding sequence of the gene.
How to predict the length of the mature gene
In prokaryotes, transcription is the process of making a copy of a gene's DNA sequence into a molecule called messenger RNA (mRNA). Prokaryotes do not make as many changes to their genetic material after it is copied as eukaryotes do.
Eukaryotes cut out and put together different parts of the genetic material, but prokaryotes do not. So, in simple cells, the finished mRNA usually matches the gene's code directly.
If the entire gene is put into and made in prokaryotic cells, the length of the final mRNA will probably be the same as the gene's code. The mature mRNA will go from where the gene is turned on to where the gene is turned off, and it will include all the instructions for making the gene's protein.