Finding a tutorial for a logo might help you learn the elements and features used for that particular logo, but Photoshop is such a complex program - you'd be missing out on many possible design ideas for your own logo.
But, if you want to give it a shot I'd check out the Tutorials section of our Industry Directory
(look for the link to Photoshop 101 and then search there for appropriate tutorials for what you'd like to do.)
As to the size - that would vary depending on if you're designing for print or web so isn't easily guesstimated. I tend to design at or a little bigger than actual size for web - for print you'll want a much larger field.