There are LOTS of similar questions and answers in this section, you may want to read some of them and come back with more questions. Basically, the old boiler is probably about 5x oversized, and your heating requirements are probably closer to 30K if you've tightened the place up and have good insulation and windows.
If you have your gas bills, you can look up the heating degree-days for your zip code, then figure out fairly closely what you need. Not familiar with the Bosch stuff, so don't know if that 100 means 100K BTU, or something else, but check that. The last thing you want is to match the old one.
On a super cold day, under ideal conditions, the boiler would be running full blast all of the time. Each time it shuts off then restarts, you're wasting energy during the cool-down/purge cycle.
A good heat loss would take longer than that. Depending on the program they used (probably one of the more generic ones), they can be 2-3x oversized. Some are within 30-40%, and if you use your actual gas useage, you should be able to come up with a figure within 10-20%.