So i have been watching alot of great tutorials for making maps in diffrent styles and stuff but i still have a problem getting started

my problem lies in that i dont know how big i want my world to be. I would like to start drawing and expanding my map as I go but is that possible in photoshop and if so how do I do it?

i thought about breaking the map up into 1000x1000 bit images and then putting them together in a HTML page but i fear that the squareness will shine through and the edges might not line up if i use filter to weather each image.