Ditto to Larry H's comments. I don't worry about DPI as 100 is more than the monitor can use. But if 150 makes you happy, it's not going too big.

For your other question, control the size using the original scan. If you make a large size scan and resize it, you will lose definition and the picture will get fuzzy.

This one is from a Mavica digital camera.

These three are scanned at 100%

On my HP scanner I have found that 62% seting for the original, gives me something very close to actual size. Below is an example. Maybe 100% looks big, but for people reading messages, it's probably better. Heck I haven't decided yet.

All are JPGs.

Nice looking chip!