You can import JPEGs fine.
Work in Pixels not physical sizes.
Too hard for me to read 😉
It all depends on your final resolution.
If you are shooting for HD which is 1920×1080 pixels, then try to keep your images above that.
If you plan on doing any Ken Burns type of effects or zooming into a face, then you have to accommodate the image for that.
You can have a mix bag of sizes just as long as each one is higher than say 1024×1024 (thats width and height).
The dissolves part Im not sure about.
They all look like shiite to me 😉