I used a 2-pixel brush in Paint.
merciary, I'm unsure whether the width of your province is reasonable, if only for cautiousness against inflationary province sizes. Other than the off brush handling, the desert could be a reasonable map bumper, as to a proper nonarbitrary reason the map doesn't expand beyond that desert. But I think it is too large.
As a Doomsday Mod I will have other players make input to me; whether they think the province width is too excessive. If they do, I might correct it.