Just as Mops has said - but I'm dead chuffed that I could have answered this question myself now!
Are they design files or embroidery files. You say it's a question about digitising, so I assume it's a design file.
A design file can be resized in your digitizing program without doing anything to stitch count or density as the original parameters are still used.
An embroidery file, however, can be enlarged in the editing and customising part of the software, but you will have to reset the stitch length and/or the density when you enlarge more than 20%.
As you asked for my tutorial I assume you have Embird. The design file would be an .eof file.
Thanks Mops, I really do appreciate the help. It is already a embroidery file so I would need to change it in customizing part.