The norm is 28 days cycle, 14 days in the follicular phase and 14 days luteal phase (lp). Lp usually ranges between 14 days to 16 days and if it is shorter than the norm, there may be luteal phase defect (lpd). What Lpd affects would be the implantation period, i.e the lining is shed much earlier than the fertilized follicle can implant itself. So if tis Lpd, will need to seek help. You will be able to gain an insight into your lp via ovulation testing. Hence, not all short cycles (especially the textbook cycle of 28 days) is not good if your lp is alright.
As for follicular phase, this is the stage whereby the follicles are grown and matured. The norm is that the body will attempt to grow multiple follicles and once a follicle is developed better than the rest, the body will stop maturing the rest and concentrate on the maturing follicle. The usual size for ovulation is aroun 16mm to 18mm and one fertility specialist said that for clomid cases, some gynae may prefer treat the follicle as matured when it hits 20mm. Another gynae said that usually for clomid cases under his charge, he'll have the patients return to obtain pills to strengthen the pregnancy. Given the usual ovulation cycle, Mother Nature will usually select the best follicle for ovulation but with clomid, it is more like attempting to mature more follicles without focus on the quality. If taking clomid, best to monitor for ovulation or do follicular monitoring to see how clomid is working for you. Side effect of clomid will be the thinning of the lining. Usually gynae will measure lining thickness too.