shaving bowl sizes

So far i have only purchased the art of shaving - shaving soap. And i have the wooden bowl. I can tell a lot of people prefer a much, i guess this is so they can leave the brush in the water and contain the later easier??

but for sizes of soap,... to me it seems a mug would require a fairly small diameter soap. So people using the mug.....do you have to be consious of this when buying the soap, making sure it is small enough??
Or do the mugs take the same size of soap as a bowl??

I took a cake of Cavendish Black when I went shopping for bowls and found lovely black-glazed ceramic ones that were also deep enough to handle enough water to soak the brush for $5CDN a pop. A pic should still be in my gallery.