Making sense of '推魏大肚'
当推:  should/ought to (应当) be/mention/select/elect/recommend (推举/首推, 指出, 是)
魏大肚：Nickname of someone whose family name is Wei (魏) and who has a beer belly (大肚).

当推魏大肚：should mention Big Belly Wei 

More example:

论这村里最聪明的，当推王小二。

Talking about the smartest guy in the village, it must be Wang Xiao Er. 

(People will all choose Wang Xiao Er as the answer to the question that who the smartest guy in the village is.)