Sidewalls are 20 feet long and depending on the covereage you perfer the amount of sidewalls needed can vary. 

Example: 

20X20 tent fully enclosed = 80 linerar feet = 4 sidewall pannels required 

20X20 tent 3 sides of the tent has sidewalls  with one 20 foot  section open = 60 linear feet =3 sidewall panels required

20x40 tent fully enclosed = 120 linear feet = 6 sidewall panels required

20X40 tent 3 sides of the has sidewalls with one 40 foot section open = 80 linear feet =4 sidewall panels required

40X60 tent fully enclosed = 200 linear feet = 10 sidewall panels required

40X60 tent with a 40 foot section open = 160 linear feet= 8 sidewall panels required

Sidewalls for your tent can keep the weather out, heat in or give you some privacy. Solid white sidewalls will keep your tent enclosed full with an entrance and an exit area.