Selecting footprints:
Read the datasheet. Regardless if they are lettered or numbered, there will be a drawing, usually with a table of dimensions for each size.
Read the footprint. View it in the browser, open it in an editor, whatever; determine what the respective dimensions are. Compare the copper area to the datasheet recommended sizes (and also download IPC-7351B and see what recommended pad sizes are for the various component types -- manufacturers sometimes botch this).
And also simply reflect on the dimensions yourself, and figure if those footprints would be reasonable to form a solder fillet, if they would be good for hand-soldering if that's what you'll be doing (usually you want some extra soldering space on the ends of the pads, for the iron tip to touch), if they aren't excessive and blobby or prone to shorting to exposed PCB areas (helps if you have the parts in hand to inspect).
No, it's not a straightforward process, and no, I don't know of any supplier that actually provides coherent, high quality, good looking libraries for any EDA software. KiCAD libraries stink; Altium libraries stink! The best are only what you personally cultivate, it seems.
Tim