The give away with cotton joggers isn't how they look in the changing room, it's what happens to the knees by week three. Cheap cotton goes baggy there fast, two loose pouches that never quite pull back, and no amount of ironing fixes it. The better ones hold their shape because there's a bit of weight to the fabric and a decent percentage of it isn't cotton at all, a touch of elastane woven through that you won't notice until month two, when everyone else's have gone saggy and yours haven't.
On fit, the thing that catches people out is the rise rather than the length. A lot of cotton joggers are cut with a high, snug waistband that sits well above the hip bone, which is fine until you sit down for an hour and it rolls over on itself. If that's happened to you, go up a size and let the cuff do the ankle fitting instead of relying on a tight waist to hold the whole thing up. Inside leg wise, I'm five foot ten and most regular lengths land an inch short of my ankle bone, which actually suits joggers better than it would trousers, since a slightly cropped cut shows off trainers rather than pooling over them. For pairing, I live in mine with a plain crew neck jumper and a parka for the school run, and separately with a fitted white t-shirt and a denim jacket for anything that needs to look like I made an effort without actually making one. They sit well under a longline cardigan too, which hides the waistband if that's the bit you're not sure about. On washing, cotton joggers are honest about their age. They soften with every wash, which is lovely for about fifteen washes and then starts to look like wear rather than comfort, especially at the seat and the back of the knee. Wash them inside out on a cool cycle and skip the tumble dryer if you can, since heat is what kills the elastane and turns a good fit into a sagging one within a season. For colours, grey marl is the easiest thing to live with by a distance, it hides both bobbling and the inevitable dog hair, while black looks sharpest fresh out of the packet but fades to a flat charcoal faster than you'd expect from how dark it starts. Navy sits in between the two and goes with almost everything else you already own, which after a few bad experiences with a mustard pair I wore twice is now the only colour I actually buy.