Preferences : Folder : Folder Behaviour : Calculate folder sizes automatically for : ...
leo, the article (it is a guide and tutorial!) is of utmost quality and sheds light on all aspects of automatic size calculation. Very helpful, really. I've read it several times. Thanks for it!
garyHal, i have large folders (with ~100000 files) on disk. so whenever i change to the root of that partition, the automatic calculation would keep my hdd/system quite busy. The setting in the Preferences dialogue prevents this (unnecessary) business. A rather more desirable function (in the Preferences dialogue) would be a setting which allows for automatic size calculation of selected items. I am missing this feature because my old (and abandoned) file manager "Q-Dir" did exactly that and liked it a lot (And other forum users had mentioned this missing feature before.), but that is not the topic of your thread.
Let us know if you like the automatic size calculation or if you eventually leave it turned off! 
Apart from all that, getting sizes for everything in a Lister's view is only 1 click away, and it is good to have that control (e.g. when i am in the root folder).