I have a 40 X 50 newly constructed Garage, that I have set up for radiant heated floor.
The 10" W concrete  foundation  was wrapped in 2" foam 42 in deep then the building was constructed using 2 X 6 studs X 12'. The outside is covered with a layer of 1/2 OSB then 2 X4 purloins were run horizontally with 1 1/2 foam between the purloins. It was then wrapped with Tyveck and then steel siding for a "pole barn" look. On the inside I am using R-19 fiberglass insulation with another 1/2 OSB inner wall.
   I have 2 Overhead Garage doors one faces west (12' X12') and one faces north (10' X 10') they are both rated a R 14.4 There are 6 windows in the shop which are double pane Low -E glass  and they are about 40" X 26" 3 on the south wall 2 on the west wall and 1 on the north wall.
There is an Office area that includes a toilet and vanity that takes up 11' X 18' area in the Northwest corner of the garage. The ceiling is R 38 fiberglass, I am guessing that the walls come out to about R 26- R28
 The floor is 5" nominal with the loops on avearge 12 O.C attached to the 2" foamboard over a vapor barrier with a 8' sand base over the clay base soil. The manifold and pumps are located in the center of the building.
The Office area is using one loop that is approx 275 ft long on a separate thermostat and pump and Eight loops in the shop area that are all about 210 ft long thru a Wirsbo Brass manifold also with a separate thermostat and pump. I had my system designed by Pexsupply and they tell me that all I need is about 49,000 BTU. I live in the Toledo, Ohio area Do thes numbers seem correct? I plan on keeping the office at about 70 and the shop at about 60 but would like to be able to heat the shop up if needed.
 I need an electric heat sorce since there are no gas lines in my area and I get a much better deal on Electricity than Propane, so what is my best bet as far as a heat sorce 

"All" you need is 49K? 49K seems high (by about a factor of 2) to me for a well insulated 2000' building in Toledo. (My similar sized, but less-well insulated less-tight house in a very similar winter climate in central MA measures-in at about 30K).

Let's assume they're wrong, and you need at most 30K- you can't get that out of an electric tank. 30KBTUs is about 9000watts- typical big electric tanks are about half that so they can still work on a 20A 240V circuit, but even those set up for 30 amp circuits are only good for around 6000-6500 watts, best case. That's still only 20-22KBTU/hr. (Which MIGHT be just enough, if they're as wrong on the heat load as I think they are, or if you're willing to let the shop run cooler than that when it's 0F out.)

You might want to break it up- heat the office loop/zone & potable HW off a standard tank (with a heat exchanger) and, run the shop off a 9-10KW electric tankless (if it'll handle the flow- not likely) or electric boiler on a separate dedicated 50A 240V circuit. (Or you could run the DHW on a 120V well insulated mini tank with very low standy like the Bosch Ariston under-sink thingys, and run all the radiant on a 9-10KW electric boiler and know that theres some chance it won't keep up on design day. There are some pretty cheap (under $1000USD) sub 10KW electric boilers out there, but I don't know how good they are, but they're not very complicated beasts in the first place.

what are the consequences of using too large or too small of a boiler?

boilers that are too large cycle more and run inefficiently as a result, and would typically need more cleaning and would have shorter lifespans. note when I say "run inefficiently" it's difficult to stress how big of a deal that can be. in some cases your operational efficiency could be less than 50%... it's a big deal.

boilers that are too small leave you cold.

It was just recommeded to me that I should use a 12KW four element electric boiler 41,000 btu. Is that about the right size? where do I get the best deal?

12kw is still probably overkill (I'm really not trusting that the heat load numbers will turn out to be that high), it may cycle a lot, but you won't be cold.

Oversizing an electric boiler isn't near as serious a hit on efficiency as it is for oversized burners, but you pay a bit more up front in oversized wiring and the equipment itself. But here I'm guessing it's on the order of ~2x oversizing, not 5x- it should be fine. Warm-up ramps will be shorter, and the efficiency hit will be sub-1%. (And if I'm wrong, and the heat calc is right, it's perfect! :-) ) It'll probably idle along on 1 element most of the season. If you get a fully modulating low-temp version (basically a glorified tankless HW heater) the lights won't flicker as much when it turns on, but it may have more buzz when operating at low heat output, depending on the modulation scheme.

I don't have enough familiarity with models & vendors of electric boilers to recommend one over the other, but they're pretty simple beasts- it should be sub-$2KUSD for any of 'em. Who stands behind them & supports them locally is probably more important than absolute price. A great deal from an internet vendor isn't so great if you have to wait hours on the phone to figure out why it stopped working, or wait 3 weeks while they ship parts from the factory via pack mule. Find a unit with local sales & support and a decent warranty, even if it's a few hundred more.