Tori Spelling & Dean McDermott -- We're Going Down ... 'Cause Renters Can’t Be Choosers

Tori Spelling and Dean McDermott are downsizing big-time -- moving into a new house that's nearly half the size of their old place.

Real estate sources tell TMZ ... Tori and Dean’s new Calabasas pad is 3,733 sq. ft. and has 5 beds and 5 baths -- nothing to turn your nose up at, but since their previous home was 6,623 sq. ft. ... this might hurt a bit.

They're renting this place, which has a yard, a pool, and a library -- and we're told the previous tenants dropped about $7k per month on it.

This is the 3rd rental home for Dean and Tori since they sold their big Encino home in 2011 ... for a loss. On a positive note: their rent keeps going down. Not so positive? The houses are getting smaller.