I don't really understand the question. If you want to post something which is intended for display on the screen I would expect that 1024x768 would be just nice. To convert it MB you would need to use the following formula
which would be 2.25 MByte
If you would use 16 bits/color the file size would be doubled, that is 4.5 MByte.
Regarding scanning I have a feeling that the following would probably apply for 6x7 scans
1) You should scan at a minimum of 1600 PPI
2) Going above 3200 PPI would in my experience give diminishing returns
File size for 16 MByte 16 bits/color scan from 6x7 (assuming size of 55x69 mm) would be
Height= 55/25.4 -> 2.165"
Width = 69/25.4 -> 2.71"
BytesPerColor = 16/8 -> 2
The formula is:
With * Height * PPI * PPI * 3 * BytePerColor / 1048576
1600 PPI -> 86 MByte
3200 PPI -> 344 MByte
The magic number 1048576 is 1024*1024. It is the size of a MByte. It may also be 1000000. The first size is the one normally used in computer science and latter one is often used when talking about size of hard disks (because smaller Mbytes make a larger number).
The normal approach is that you scan at the highest resolution you have access to and scale down to the size needed (using "Bicubic sharper?" in PS). You always need to sharpen after changing scale.
Any suggestion on what file size I should have scanned to???