Well. Females are generally larger than males. ( im sure there are some exceptions)
There are also many different species. Some get larger than others, some are smaller ( Dwarf Island Locales)
I think a great starter boa is a Caulker Cay island boa. they stay small, are very beautiful, and very docile ( but of course, remember that every snake is different, I have two and both are angels!)
They are a bit more pricy than your common redtail, but they are much much smaller and easier to manage.
Snakes, if well cared for, do not smell. They might have an odor when they defecate or excrete urates ( Hard white stuff, that is their Pee * snakes are very well designed to use up EVERYTHING they eat and drink!*) But if you clean it up right away, you shouldn't smell a thing.
You can feed your snake Live or Frozen Thawed Food, and its best to ask the breeder or store you are purchasing from whether the snake is eating either or. and to demonstrate, or show pictures of such feeding.
You would also need an appropriate cage for a boa. More arboreal species might need more height in their cage, than length. while ground dwelling species, might need much more room to crawl and stretch out their body nearly full length.
On any tank you get for whatever species, you will need to have a hot end and an cool end ( horizontally, vertically, maybe both ways for some species) This can be achieved by a heat mat with a thermostat ( a MUST) and or lamps. This helps achieve a thermal gradient, so your snake can thermoregulate and it helps for digesting a snakes food. ( very important!)
Youd also need to keep the correct humidity for whichever species you keep as well, so temperature gauges ( digital) with a humidity gauge is a good idea. Some are relatively cheap that also come with probes that you can place in the warmest area of the tank, to better monitor your snakes basking areas.
There are many other species of boa, and some might be more difficult to care for than others. Research a bunch and see what you like!
As I said, research research research!!!

And good luck in choosing a new Boa!