Standard Sizes of Sofas. Sofas vary in size, from 6 to 8 feet long and 32 to 40 inches deep for a standard size sofa, to 4 to 6 feet long and 28 to 30 inches deep for a loveseat.The actual depth of the seating area is reduced by the depth of the sofa's arms and back of the sofa.ofas vary in size, from 6 to 8 feet long and 32 to 40 inches deep for a standard size sofa, to 4 to 6 feet long and 28 to 30 inches deep for a loveseat.