How far of a train ride from koln to bonn? Also how large are the lockers in the train stations in general?
In Cologne you'll use this instead of lockers:
Luggage storage system
Not sure just how big lockers are in Bonn. They come in different sizes.
Generally 25 to 30 minutes on regional trains. Transport is controlled by the Rhein-Sieg Verkehrsverbund (hereafter called the RSV). Prices currently show 7.70 euro one way for the 4 zone journey. You can also get unlimited day ticket for about 2.25 times that, or a minigoup ticket for up to 5 people for 3.1 times that amount. There appears to have been a 8.5% price increase since the published rates on their website. The listed single ride ticket is listed as 7.10 euro (as of 1/1/2012). Even Germans may fall behind on updating websites. http://www.vrsinfo.de/englisch/the-vrs/vrs-about-us.html
Koeln has an automated luggage storage system. Bonn just shows lockers.
You could also ride a mainline IC train for 11 euro or an ICE train for 15 euro for a 19 minute trip.