Depending on the program the quality will be average to poor.

The size online depends on how many pixels are in the image. If 5x7" photo is scanned at 100 pixels per inch, there will be 3500 pixels in that image, with 500 pixels wide x 700 pixels tall. Even phone screens are now larger than that, so the image will look small.

When I scan images for archival purposes I scan as large as I can and then resize as needed for various purposes, saving the original high resolution scan as I would a negative. For online use, I generally resize to 1024 pixels in the longest dimension and add "_web" on the end of the file name. For printing I want at least 300 pixels per inch depending on how large I want to print the image - for 8x10" that would 2400 x 3000 pixels.

For archival purposes, I scan 5x7" photos at 1200 or 1600 pixels per inch, 35mm slides at 3200 to 3600 (depending on the scanner). I also save them as TIFFs which are not compressed. When I resize for specific purposes I generally save the edited files as JPGs which ARE compressed and can be much smaller.

I am not sure what you can do to make your images larger and keep the quality. You may need to re-scan the originals.

150 to 200 dpi is usually OK for a document to be emailed.
Try scanning at 300 dpi or even 600 dpi and see how you go. Remember that if the document is small or poor quality, you can't make it much better, but you can increase the size by increasing the resolution.