Whew, they are on opposite ends of the spectrum.
I personally LOVE lovebirds

I currently have two and recently found another one in a tree. And let me tell you, all 3 are night and day different.
Lovebirds can be kept both singly AND in pairs - however, I believe it is a more personal decision and I would base it what you may want in a relationship with your bird.
I had Skittles by herself for a year (she has been "paired" with other lovies before) and she was (and still is) the most snuggliest, hands on and loveable bird I've ever met. A year ago in March, I bought Marley. Not because I wanted a companion for Skittles but as another addition.
Lo-and-behold, they ended up getting along GREAT, Mar tries to hump Skittles and feed her, but Skittles just doesn't care, she would rather be snuggled on me until she dies.
What I'm trying to get at is:
• Singly kept lovebirds
usually tend to have a more bonding relationship with their human counterparts
• Paired lovebirds have each other to rely on so they rarely tend to seek that attention from us humans
But keep in mind, there are ALWAYS exceptions to every "rule of thumb".
For cage sizes, obviously, the larger the better. I have two large cages for S&M however, they usually cram themselves into one happy hut sitting on TOP of the cage haha.
I LOVE this cage and will eventually get it haha:
BIRDCAGES4LESS.com: Tiki Treehouse™
However, my two larger cages are very similar to this (don't hold me to the dimensions though)
BIRDCAGES4LESS.com: Kiki Kottage™ - Two Top Options!
I will measure both cages tonight for you.
Overall, if this is your first bird, I would get one

See how your relationship develops with him/her over 6 months and then decide if a second one would be the right move.
I've had Skittles for almost 6 years now and I know she'll get along with ANY other lovebird, so I'm not worried about ones that may come and go. She has been the way she is today, since day 1 of bringing her home.