Ummmm... Welll.. Ok. lets see if i understand..
(
if this is the problem, i doubt it)
This is how you need to rezize it to fit your window. If it does not fit the screen (IE: you see other things on your screen with IE is up), you need to hit the maximize button on the browser.
It looks like this;

(
Or this is the problem, possible)
If you just wante a bigger screen size, you need to go into your display properties and adjust it like this.. (assuming you have windows)

..
But keep in mind everyone has different display sizes and screen sizes so what might look right to you, may not look right to everyone else.
(
or this.. very possible)
SO if you are concerned about you images being too
BIG then size them to be
smaller. Then everyone will beable to see them in whatever they are viewing.
I would say make whatever image you are trying to enage no larger than the suggested 300 x 300 px. But you can definatly go larger, probably up to 400 if you wanted. But if you are that concerned, i would not go much more..
Theres my 2 cents!

-J