Microworms can mean "Microworms' but it can also be used to describe a group of similar sized "worms" like Micro worms, Walter Worms, Banana worms, and even vinegar eels. All make great foods for fry.
IME they don't smell unless you don't re-culture often enough. Microworms need to be recultured about every 2 weeks or so to keep the culture fresh. When the culture is kept up I find it has a pleasant, sort of musty smell, not a great smell but not horrible. If properly contained (in a plastic container with a lid and air holes covered with paper towel) you won't smell it at all except when harvesting the worms.
My favorite size tank for spawning is 5 gallons. I've left the fry in that size tank for up to 4 weeks before transferring to a larger tank. This time around I'm transferring them around 7-14 days old because I think I'll see better growth. I'm going from a 5 gallon (filled) to a 10 gallon and then will start adding about 1/2 to 1 gallon of fresh water a day, once the tank is completely full (which will take around a week) I'll start doing 50%+ water changes daily.
When the spawn seems big for a 10 gallon (which I'm guessing is going to be somewhere around 6-8 weeks, maybe sooner) I'll transfer them to a 29 gallon grow out which is the largest tank I have right now.