Bathroom fan sizing in 2 room bathroom with always open doors
You shouldn't have to count the room with the sinks, but you may want to.  If you find that the mirrors in that area tend to fog up, during normal use of just the sinks. Then you might want to include the area in the calculation.

400 ft³ at 40' EDL, means you need at least a 60 cfm fan to meet the minimum air changes per hour required by International Residential Code (and others).  You'll also want to make sure you run the fan while you shower, and for 20-30 minutes after you're done. A timer unit is the best way to manage this, but you'll have to make sure you get one that's rated for motor loads.

Every time you exhaust conditioned air out of the house, you're losing some efficiency. However, in the case of steamy bathroom air, it's worth the lose to reduce the chance of mold and mildew issues.  You want to exhaust the moist air, but you want to exhaust as little conditioned air as possible. So it becomes a balancing act between wasting conditioned air, and preventing mold/mildew.