About 4 and a bit inches.......had them both since tiddlers. The common (£2.99 from P@H ages ago) will outgrow the tank eventually but hopefully the Cochliodon will be able to stay (had it for a few years now)
looks like a common to me. ive got a rusty and and a sailfin in my tank, same as yours they generally stay at opposite ends of the tank, they do get on but seem to choose not to see eachother haha. im proper lovin my rusty.