Odd problem with the lasso tool

I have finally figured out a way to get rid of my background and turn it white.  I'm using the Magic Eraser tool.  After going all the way around with the Magic Eraser, I need to "lasso" the image I have marked with the Eraser, then hit "inverse", delete, which leaves me with a blank background.  Here is the problem.  When I have the image open, I can't see the whole thing if it is long and thin and need to extend the Lasso tool down below what I am initially seeing.  But if I get to the bottom of what I am seeing, the image does not "roll" down so that I can access the rest of the image.  Maybe I have something set wrong?

I had to reduce the size of the image so that I could see the whole thing and then use the lasso tool.  This worked, but when I saved the image, it was the smaller size, would not have default back to the original size that I started with.

I'm new on this Mac and PSE15 is also new for me.  Have I set something wrong that prevents the program from allowing me to roll down by pushing on the bottom of what is visible?  I can't let go of the Lasso tool.  It has to be used continuously around the whole image.

I'm stuck!

thanks  and Happy New Year!!

jolie