Red swords are overrated by new fighters and underrated by experienced fighters. They're harder to learn than new people think, but can be used quite effectively if you actually know what you're doing. Find someone who does and have them teach you - most experienced, dedicated red-sword fighters are happy to have more people out there showing that their weapon isn't just for n00bs.
Weapon lengths is very much a matter of trial and error, personal preference and fighting style. Personally, I would say to err on the longer side (5'). With a 4' red you have to have
very fast arms, good footwork, and good pre-emptive parrying because you don't actually have that much reach on people - while you may be in range for a kill shot well before they are, your
arms are very much in range, and a one-armed red fighter against a shieldman is in pretty dire straits. A buckler can help a
lot with that.
I'd say see if you can borrow someone's red, fight with it a while, have someone who knows what they're talking about show you the ropes, and then see how you feel. If that's not an option, go for 5'+ - it's a lot easier to make a long sword shorter than a short sword longer
.
Oh, and don't forget the stabbing tip. Stabbing tips on reds are a wonderful thing indeed... .