What is the width and length of a LDS cultural hall?

Answer 1

Not all cultural halls are the same size, the size of the cultural hall is based on the size or 'phase' of the building. The current floor plans used in the US and Canada by the Church of Jesus Christ of Latter-day Saints have the following cultural hall sizes:
Modular: No Cultural Hall, chapel space can be multipurpose/banquet as needed (seats 40-90)
Independence 50: No Cultural Hall, chapel space can be multipurpose/banquet as needed (seats 30-60)
Independence 90: No Cultural Hall, chapel space can be multipurpose/banquet as needed (seats 80-140)
Independence130: No cultural hall, chapel space can be multipurpose/banquet as needed (seats 80-160). Approx. 42ft x 42ft including rostrum.
Independence170: Partial (half) cultural hall, 1275 sqft, approx. 42ft x 33ft
Independence230/300: full cultural hall, 2530 sqft, approx. 42ft x 57ft
Independence230/300 Stake Center: 2530 sqft approx. 42ft x 57ft
Heritage98: 2350 sqft approx. 42ft x 64ft w/o stage
Heritage09T/14T: 2560 sqft w/stage approx. 65ft x 42ft w/o stage
Heritage09T/14T Stake Center: 2560 sqft w/stage approx. 65ft x 45ft w/o stage
Measurements were not available online for older (pre-2006) standard plans, including the Legacy, Palmyra, Fayette, and Sharon designs. Additionally, not all meetinghouses follow the standard plans.

Answer 2

They're not made to a specific dimension ! Many LDS halls are just old buildings that haven't been used in some time.