Well you have a choice between boids (boas and pythons) and colubrids (milksnakes, kingsnakes, ratsnakes, etc). Boas are usually heavier bodied, slower moving, less active than colubrids, which are more slender bodied, quite active and can be a bit high strung, as well as they defecate much more than boids.
Spotted Pythons usually finish up around 3'. They are relatively easy to care for and are usually quite docile.
Childrens Pythons are similar to Spotteds, although their pattern fades and they have a slighlty different colour. They are also a bit smaller.
A Hog Isle Boa can also be a great choice. They are easy to care for, and usually pretty docile once you get them out of their cage. When they are in their home they have a very strong feeding reflex. They average around 4.5-6 feet in length, some females may occassionally exceed this. Below pictured is my 6.5+ foot female.
Central American Boas are great as well, depending on locality... they range from 4-7 feet in length. They have the same care as the Hog Isle (they are the same subspecies), though they can be somewhat more variable in their temperaments.
Rosy Boas usually max out around 2-3 feet. They come in a variety of different colour and pattern variations depending on the locality.They are super easy to care for and tolerate handling better than most species. They are probably the slowest moving boids you will find. They don't get stressed very easily either. Below pictured is one of my mexican locality Rosy Boas...
Sand boas can also be a great choice. They range from 1.5 to 3 feet in size, and their care is similar to that of rosies. They are usually pretty easy to handle as well.
Rainbow boas can be a good choice if you are prepared to for their lower temperature/high humidity requirements. As long as these are met they make great captives. Most commonly available are Brazilians and Colombians. Both are usually great feeders and easy to handle. Colombians are shorter (4-6 feet) and stockier, and dull in colouration. Their care is similar to the Brazilian, though they can tolerate lower humidity better. Brazilians attain larger sizes (5.5-7 feet) and are, IMHO the closest thing to having a guaranteed "tame" animal. Babies can be nippy but calm down fast and easily and stay that way. They aren't a very stocky snake so a 6 foot rainbow is nowhere near as large as a 6 foot Hog Isle. Below is my 2 year old orange male...
Corns reach lengths of 4-5 on average, with some specimens reaching lengths of 6 feet. They are usually handleable, are very hardy, and are great feeders. They come in an endless array of colours and patterns.
Milksnakes are easy to care for however can be a bit spastic in terms of handling. Not so much that they bite as they musk, defecate, and urinate all ove the place. They reach lengths of 4-6 feet. They are very nervous and super fast, but can be very pretty.
Kingsnakes can be similar in milksnakes in terms of musking all over the place. They are easy to care for, but again can take a lot mreo effort to acclimate to handling. They finish up around 4-6 feet as well. Some species such as grey bands may be more difficult in terms of feeding.
Bull snakes are easy to care for, and usually reach around 6 feet. They are relatively easy to handle as well.
Some ratsnakes such as grey rats, everglades rats, and black rats are relatively easy to care for and can become handleable with some time. They attain sizes of 4-7 feet depending on subspecies.
NOTE===Although this species is commonly recommended as a beginner snake, I would have to disagree. Ball Pythons are IMHO better as second snakes. They can be finicky feeders sometimes, and often go on fasts, which can be very frustrating for the novice keeper. In addition they are a very timid snake and can stress very easily, which doesn't make them a great candidate for frequent handling.
Good luck with your decision. Make sure to research thoroughly and have everything setup before you are ready to bring your new little friend home