My RV side of the shop is approx. 46 feet deep x 22 feet wide. I preferred to have a deeper car bay than wider car bay so an RV would fit (for resale value). The walls are approx. 15 feet in height. The single car bay side is 26 feet deep x 20 feet wide with 10 foot walls. We placed the slab 1 foot below the top of the poured stem wall for additional height. The roof is peaked pretty steep for additional ceiling height. That gave me plenty of room in the loft storage area above the bathroom.
I prefer the two post lift for versatility mainly. You can remove tires/wheels for service w/o using a jack bridge as on my four post. There is more access to the undercarriage. You can leave the car on the two post lift to take weight off the suspension for extended periods of storage. A two post lift is generally capable of more weight than a four post. You can place longer vehicles on the two post lift than on a four post. The only real advantage to the four post in my opinion is the convenience of driving on to it and lifting the car w/o having to get down on the knees to adjust the arms.
I would strongly suggest that if a lift is in your future, start gathering brochures from companies now and study the vast differances in them. For very little additional money, a commercial grade lift is a better deal over a "hobby lift" if you plan to use it extensively (and you will). Your friends will also want to use it. Make sure that the company uses structural steel and has had their lifts tested by the independent testing facility (usually located in KS). My previous two post FORWARD MANUFACTURING lift was 9,000#. but it was a commercial grade lift and tested to twice that amount of weight. Most lifts only require 4" of aged concrete, but if you go to a commercial grade lift, use no less than 6-8" of steel reinforced concrete beneath the footprints of each vertical column. The vertical columns on my two post each weighed over 1,000# and I chose to have it delivered and installed by a distributor for just $300.00!