To ensure that the maximum amount of detail in your painting is exported, be sure to set the appropriate canvas resolution at the very beginning. For example, when using Ambient Design's ArtRage app, you have the choice of increasing the canvas size to a maximum of 2048-by-2048 pixels—double the native iPad display resolution.
Some apps don't offer this option or require the processing power of the iPad 2 to enable the larger canvas options. Other apps allow you to create artwork that is resolution independent so that your art can be scaled larger than the size at which it was originally created.
Taptrix's Brushes app offers a companion Mac application called Brushes Viewer, which can essentially re-render your paintings at up to six times the original size. Other resolution independent applications include InkPad and Adobe Ideas, which produce infinitely scaleable vector artwork.