You might have to take the ecu out and do it the regular way. Drill it out with a drill out. First you drill a hole in the center of the bolt and then use the drill out to get it out. Looks like a drill bit with reverse threads. You screw it into the hole and when it catches it will unscrew the bolt. Unscrew by hand. Don't put the drill out in a drill and hit the power. That's probably a poor description but the drill outs are what you need if you can't get it out any other way. If you don't feel comfortable, take it down to a machine shop or to a mechanic and they will get it out.
For figuring out bolt sizes I picked up a few things the other day that are useful. At pep boys I found a little tool that measures thread pitches. They had one for metric and standard. And I got a little handle held caliper to measure sizes. For example, say the bolt is 12mm x 1.25. Measure the bolt diameter with the caliper. 12mm. Then use the other tool to measure the thread pitch. 1.25. Works great. What I did in the past was take the bolt down to lowes or home depot to find the size. They usually have a size chart where you can screw nuts and bolts into to figure out the size. Then go get your tap.