Generally the bigger the spider the bigger the cage. Terestrial spiders should have short cages and aboreals higher cages. Terestrial spiders can easily bleed to death from a short fall, aboreals on the other hand are more dextrous and can survive longer falls. In general the container should be at least 2.5 times as wide as the spider is long. Care should be taken when choosing decorations because some could be harmful to the spider.
Plants are very nice additions but some are potentialy harmful.
Cactus for example could empale a falling spider. For most species a shelter of some kind should be provided. Cork bark works well because it won't mold. Other possibilities are half of a clay pot or anything they can hide under to get out of the light and heat. For aboreal species broad leafed plants work well also.