It's always better to make things like patterns and brushes too large and then scale them down because going the other way leaves the edges jaggy. There is no typical scale and sizes vary as well. For the purposes of our forum's software there are limitations to the size of image that can be directly posted. Generally, that size is a maximum of 4000 x 4000 pixels but there is a further restriction of file size which is 4.7 megabytes. When saving as a jpg in PS you can turn down the quality of the image to reduce file size or you can just resize the image to something smaller (again going up produces jaggies). For larger images you can always zip them or host the image on another site and provide a link (like Deviant Art for example).
I usually use the high end of the limitations so that I can squeeze in as much information as possible but other styles look very nice done smaller...so that is up to you. Maps like battlemaps that would be used in Virtual Table Tops can be any size so long as they fit certain aspect ratios of ppi to feet (you can search the forums for posts on what those scales are).
If you are referring to the content of the image then we use a rough outline of Worlds - Continents - Regions (as big as a nation or as small as a county) - Towns/Cities - Dungeons - Structures/Buildings/Ships. For a world map you might have 1 pixel = 5 miles and for a battle map you might have 1 pixel = 2 inches...it's all up to you. Those of us who post non map artwork, like people and drawings, have no regard for pixels whatsoever so scale is thrown out...we just make it up
The only real things you ever have to consider is dpi (dots per inch) if you want to print something (300 is good, 600 is better) or ppi (pixels per inch) if your maps are only going to be seen on monitors (for that 100 is good enough).