Bigger maps does not necessarily mean a slow down its just generally true due to the way they have programmed the app. I have done raster maps up to 40K using about 30 layers but you cant generally do it with the usual apps and methods. The main thing I would suggest is to tile your map and deal with one tile at a time - much like doing one continent as Kodiak says.

I would suggest first looking at what res you need. I expect that you need a certain resolution at some points of the map and higher in specific other areas and probably much much higher in cities etc - a resolution which is completely unreasonable to map the whole world at. I.e. Although you may need quite a high res for the world map its probably not 40K sqr. I don't think there are any standard pixels per mile type resolutions and in any case some apps will cope with a variety of resolutions and scales.