Also keep in mind some of your wife and kids things, like honey lets put the xmas decorations in the shop or dad can we put our bikes in the shop or dad can we make a play house in one corner of the shop. I have an old 30x30 and am going to tare it down and build a 30 x 70 with a 12' lean to across the back for lawn mowers and bikes and extra motors/trans/parts. The 30x30 portion will be two stories and the wife and kids can have the upstairs area as long as they don't mess with all my small parts that will be up there like carbs/distributer ect.
The other 40x30 will have 12' ceiling as mentioned above with a two post lift.