Not sure of which version of SONAR you are running, but X3 and newer you can launch SONAR from right clicking a MIDI track (use "Open with...") and when SONAR opens it will automatically load TTS-1. You can insert a soft synth and shift-drag the MIDI to another synth easily.
Also, doing the above will import the MIDI file's tempo and set the SONAR project accordingly. I suspect the "shrink" you speak of came from importing a MIDI file into a project with a faster tempo. In that case, the MIDI will play faster than anticipated. You can also go to the beginning of the project and insert the proper tempo that the MIDI file was done in (and will only be simple if all the same tempo). If you have multiple tempos in the MIDI file, consider launching SONAR as mentioned above and working from there.