... (cont.) Resources:
http://www.stadt-koeln.de/koelntourismus/sehenswuerdigkeiten/kirchen/index.html -- a directory of churches in Cologne. Knowing German helps.
http://www.romanische-kirchen-koeln.de/ gives (menu point 'Kirchen') a useful graphic representation of churches in relative scale.
Regarding the size of the building: I couldn't find a reliable internet source regarding St. Martin. For St. Aposteln, the largest romanic chruch in cologne, a hight of 67m for the tower is given. An over the thumb estimate with of the churches' relative size leads to ca. 50m for St. Martin (tbc)...