The reasons I went with the 155,000 btu boiler:
After multiple designers and installers looked at my building project they all came back with multiple ideas. With the r21 in the walls, r38 in ceiling, 2 r17 16x14 overhead doors, 1 man door and no windows the heat loss was calculated and the 155,000 btu boiler was deemed sufficient. I realize the "double bubble" insulation under the slab doesn't have a high r value, but I had multiple people that have buildings with slab heat say a high r value isn't really necessary. Something that reflects the heat up was more important. Hopefully this is correct.
After listening to designers and engineers I always like to look at real world applications and see what has worked for others. It seems that some engineers like too over engineer floor heat, in my opinion. Some want 6" spacing for several runs all around the outside perimeter and in front of doors and others say it doesn't make much difference.
I have multiple friends with buildings that floor heat. One has a 60x195x16 that has 1" foam under his slab with a 140,000 btu boiler that is 10+ years old. He only has tubing in the front 60x85 and I've never noticed the back 30' being a different temperature. It is always comfortable in his shop. Another friend has a 60x80x16 with a 135,000 btu boiler and his works great.
Luke....you kinda answered you own question when you stated that the engineer said you needed a 100,000 btu for your building, but your 75,000 btu furnace is doing great at 71*.
I will keep mine in the 58* range. I can't say how mine works yet, but I'm hopeful that it will do fine.
The installer got a some more of the "works" mounted today. I think it's coming along nicely. I took some blank electric covers and cut a hole and put them around where my 1" air lines come thru the walls to give it a "cleaner" look. I like the way it turned out.
Thanks everybody for the kind words and questions. I realize not everybody agrees but don't feel bad about saying something if you don't like or agree. I'm sure there is something in your garage/building I don't like.
If your in Ohio I figured you'd of used the tri-ply post with the treated lowers. My 6x6 post are 5' in the ground and the first 8-12 feet are treated then stagger out to regular 2x6's (all triple stacked of course). My longest posts were 32' long if I recall. 17' truss height. 4-12 pitch makes her about 26' tall +5' in the ground. 
What's the BTU rating on that bad boy!! 
