While you can place the drain anywhere, the preferred location is centered. The floor of the shower needs to be sloped towards the drain from all sides. You determine how much lower the drain is based on the longest distance water has to travel. Say 4', it must be 1" lower than the walls. Dropping 1" over that 4' is a nice gradual drop, but if the drain is only 1' from the other wall, it still has to be 1" lower. Now, you have 1" over 1', a quite steep slope. (This is assuming you want the bottom row of tile to be even all around which is what is normally done.) So, now you can decide. Check out
www.johnbridge.com and read up in their 'liberry' section on how a shower is supposed to be built. It will explain things, and help you know if yours is being built properly and maybe help you decide how you can prevent problems in the future. It's a big expense, and should last decades if done right...dig out the knowledge, it's worth it.