I may have to get back regarding photo size, something about reducing pixels?
For me, having the larger dimension of the picture at 800 pixels is usually good.
Allowed photo file size is pretty generous on Stunthanger, so posting much larger pictures is possible. If you are posting something extraordinarily nice, a larger photo lets us see it in better detail.
My computer came with Microsoft's Paint program which is pretty crude but will do for resizing if it is all you have. I have another photo editing program but I don't remember if it came with the computer or with the camera. It seems to make some adjustments so that picture quality doesn't suffer during resizing so much.
If you resize a photo, you might want to first make a copy of it (Save As <different name>) and edit the copy. That way you don't lose or wreck the original.