In that size range, either a rainbow boa or one of the smaller locales of
Boa constrictor might be good options. Rainbow boas under a year old are a bit more challenging to keep healthy, as they need more careful control of moisture and temperature than other species; an older captive bred one will be more tolerant of error, though still not the absolute easiest species to keep -- spending what it takes to get quality housing (PVC cage, quality thermostat, good heat panel; tubs for smaller species work well) helps a lot. In general, plan to get the best purpose designed enclosure/heating you can for the species (no fish tanks or ExoTerras) for best results. Stay away from bioactive, UVB for species that don't need it, and other contemporary bandwagon-jumping movements.
Oh, yeah, recommendations.
Ball pythons are in that size range, too. Rosy boas are smaller, three feet or so max. There are many other species in that size range, but sticking close to what you mentioned for now.
My ball python is good natured if boring (boring is kind of nice, sometimes); my rainbows are sweethearts if you respect their space and don't bother them when they're not in the mood (and even then are very hesitant to make their displeasure known) and my rosy boas range from sweet little things to food-driven maniacs.
Generally, female snakes are a bit larger than males but not by too much.
Going to reptile stores and expos can be informative, though leave your money at home on your first handful of visits and double check anything anyone tells you there on a quality forum (you'll get good advice here, and the
ball python people have a lot of knowledge about a range of snakes, too).
Once you start asking questions about a certain species, you'll learn more about what you're looking for in a snake ("oh, they sometimes bite? Not for me") and can get more relevant suggestions -- so keep asking.