The crib mattress that fits in a toddler bed is 52 inches long. so a child could fit in the toddler bed until they were 52 inches tall. Most kindergardeners are 42 inches tall.. so they will fit in the toddler bed for a long time.
My daughter is 4 and 41 inches..(tall for her age) and she fits in her toddler bed.. she will stay in the toddler bed as long as she can.. her room is smallish and the toddler bed fits better..
PS. a full size crib is also known as a "6 year crib" as most 6 year olds will still fit in a crib... so most 6 year olds will fit nicely in a toddler bed..