Please explain more clearly. What is the pixel size and dpi that you are using, and which file format are you saving to?

I keep all art saved as a Sai format until I need to upload it to here and then I save it as a JPG file. I .. don't know much about the files and formatting so that is why everything has gone array.
Erm.. I use the preset of height 2500 and width 2000.
The Saint Patrick's Day picture was done on H: 800, W: 600. I Hope.. yeah that made it a good size and not overly large but when I try to re-size and redo the resolution to the other pictures like the WIPs, they wont re-size properly and I am just so confused. This is the first time I have ever used programs and such or a tablet so I really have no idea what I am doing. I have just been winging it... -hah, accidental bat puns- So I don't understand or know how to .. re-size or reformat them to not make the picture too big for the screen.
I am not good at explaining but I hope you get the jist of what I am talking about.