I would create two images. Resizing with PHP on every view adds server overhead. (You could of course write a script to resize each image only once and store the resized version separately until the original was modified again.)
And how does that affect the size (in bytes) of the image file?
Not sure I understand the question. If you're asking how dimensions affect image file size, the answer is plenty - but exactly how much depends on the original size and the target size.