just text is generally small, one hundred kb maybe for a text email but these vary greatly depending on length. Throw images in there and you might be looking more at maybe up to half a mb email, probably less. Attachments change it again so it's really subjective. How long is an email? I'll tell you if you can tell me how long a piece of string is. Best thing to do is experiment, see how much data you use, look at the length of your emails. Maybe even just hang it out til the end of the month and giffgaff will advise you the best deal for you in your monthly statement and you can see you data usage for yourself.
*Edited, I overestimated the size of the emails