As Alice in Wonderland discovered, physical size can be extraordinarily important. It can often be just as important with two-dimensional art. Some subjects clamor to be large; some are content to be smaller and more intimate.
An example of a large painting that needs its bigness is The Diver by Jasper Johns. Over 7 feet high, it's a dark presence that overwhelms in part with its size. To me, it needs to be seen in actuality to be appreciated.
When you look at paintings on the computer or in a book, any shrinking in size should be taken into account. Not only is impact lessened, but big paintings will appear both tighter and busier than they really are.
The painting above is a tiny 8x8 inches and wouldn't work as well if made much bigger. If I do another using the same reference, it would be both larger; and I think quite different in its mood.
Nimbus is an 8x8 oil on panel done during the Karin Jurick (master of the small) workshop in NYC.