AFAIK the classic view is not customisable in that way.
20 messages at a time in Nabble seems very reasonable; when I compare with Mail (Mac OS X) the limit without warning is 10.
Topics such as <
http://n2.nabble.com/-td2306610.html> and <
http://n2.nabble.com/-tp2692753p2692753.html> contain messages that are relatively short, and it might be reasonable to wait for all messages to appear.
Other topics such as <
http://n2.nabble.com/-tp2297334p2297334.html>, <
http://n2.nabble.com/-tp2379087p2379087.html> and <
http://n2.nabble.com/-tp2305721p2305721.html> might take an unreasonably long time to load in their entirety.
Defocusing from Nabble: whenever I want to view more than one page at a time, I open each page in a tab.
Regards
Graham