That's a bit vague, you have to tell us what the name of the program is that you are scanning with so people who know that program can help you better.
For the time being don't select BMP output files but JPG or PNG instead, BMP is the largest filetype of the three.
If the documents are black and white, during the process of prescanning, if you can select Acrobat files PDF select that and set it to black and white scan. Those should be really small, even if you scan five pages or so.
Leave BMP alone, that is for graphics.
If your documents have color and you want color, PDF will do color too, but the file sizes will be a bit larger, maybe around 1 meg to one or two pages. Again, you can always fudge around with the DPI settings, if they are set really high, like 600, bring it down to about 150-200, that should be readable.