I've been asked this question several times, so today we're taking a break from regular programming (whatever that is) for me to show my astounding blogger expertise. (I am kidding about being an expert - please don't send me hate mail for making false claims and being an arrogant moron. Its sarcasm, my dears)
So, how do you make your pictures bigger in blogger? Its really quite easy.
1. Adjust your main column width to a size that suits you. Mine is 550px, which means I resize all my pictures to be 550px in width. You can find out how to adjust the width of your columns by people who are much smarter than I am - this tutorial is a good one.
2. Once you've adjusted the width of your picture to match your column width, upload it into blogger and go to the Edit Html tab:
3. When you click on the tab you will see something similar to the following, which makes up the html code for your image:
Its the info highlighted in green and red that interests you. Simply delete all of the green code, so that the line looks like this: style="CURSOR: hand;" Its important to leave those quotations and semicolons intact. Then go to the red text, and change it to say either s640 or s800 (the difference is how big the picture will actually be). I use s800 for my pictures.
I am sure there are a lot of better ways to go about it, and if you use flickr or another image hosting site I think its less complicated. But that's how I go about resizing my pictures in blogger - hope that helps!