Generally the size of your grow space determines what would be an ideal plant size for that space. In a 5x5x6 tent you will only have about 4 feet of usable vertical space because of lights. Possibly less since you don't want you glass sitting right on your plants. Now plants that are vegged for 3 months could very likely outgrow that space with out careful control of their height.
I would say that typically perpetual grows are done with larger numbers of smaller plants. You don't spend time vegging plants for 3 months, you veg them for 2 weeks, and get them flowering. Yes the plants are smaller so they don't yield as much per plant, but if you have 2-4 smaller plants instead of 1 huge one you are able to yield about the same.
I've been running my garden like this for 6 years now. I take clones about every 2 weeks when they are rooted they go into pots, and veg for about 2 weeks until they are roughly 8 inchs tall and growing vigorously. At this point I've already cut my next batch of clones as well. Once the potted clones have vegged, they go straight into flower for 9-10 weeks depending on strain. Then every 1-2 weeks I have anywhere from 8-12 plants to harvest. I average about 20g per plant, and can fit around 45 plants under my single 1k light at a time.
Though there are many ways to make a grow perpetual, I feel the way I run mine is a good example of a typical cycle. Its done to increase the number of harvests per year, and to decrease the amount of time spent vegging the plants instead of flowering them.
