One more opinion, from a 50 year old who wears "coke bottle" glasses for nearsightedness and astigmatism:
1st: Should I be looking for a 40 or 60 sized plane, and what are the advantages of one over the other?
40 size because you have a 40 size engine. My first trainers were 60 size but would have been 40 size if I already had an engine. I now have a 40 size trainer that was given to me by a club member.
2nd: I have a pickup with a 5 1/2 ft. bed, which will possibly limit me on the wingspan of whatever I buy. Are the wings for trainers transportable in 2 halves, or are all trainer wing halves glued together into one long wing?
Most are one piece. Most 40 size trainer wings are 5'0" to 5'3". As already mentioned, even a six foot wing will fit diagonally.
3rd: Are there any recommended trainers that use wing bolts, or should I stick to rubber bands?
Bolts or bands doesn't really matter. These will work with your engine and were "in stock" on June 23.
These use bolts:
http://ecsvr.com/abm/shopexd.asp?id=2925 $99.99
http://ecsvr.com/abm/shopexd.asp?id=40 $109.99
These use bands:
http://www3.towerhobbies.com/cgi-bin...?&I=LXCAS2&P=0 $69.99
http://www3.towerhobbies.com/cgi-bin...?&I=LXPY56&P=0 $109.98
http://www3.towerhobbies.com/cgi-bin...?&I=LXK971&P=0 $109.98
http://www.horizonhobby.com/Products...ProdID=HAN2055 $79.99
4th: As for the engine, I'll buy whatever size is recommended for whatever size plane I get. However, I happen to have a VERY, VERY old (25+ years) K&B 40 that is basically new, with probably only a couple of hours on it, if that. It has been properly stored, so it should run fine. If I end up buying a 40 sized plane, would this be okay to use, or should I just buy a modern .46 engine instead?
Save your money and use the K&B.
IF you decide it's not powerful enough you can always swap it out for a 46
5th: I do have a NIB Saito FA62a that I bought along with a Pulse XT 40 ARF. Should I run the Saito in my first trainer, or should I save it until I am skilled enough to fly the Pulse XT?
Keep the Saito new until you're ready to fly the Pulse.