welcome to the forum!
you will find a wealth of information here i think will be helpful. first of all, the search function at the top will most likely turn up lots of threads on speakers and installs, always try searching first.
if your Disco is anything like mine, it has stock 4" all around. not sure if it is or not, but someone will chime in about the 92 year model. if thats the case, 6" probably wont fit up front due to the door handle being in the way. yeah, 6x9s would fit on the rear pillars with some work, but i would go with 5.25 or 6.5 at the largest. one more piece of advice...take it to a shop that knows what they are doing, tell them what you want, and let them do it. sure it costs a little more in the end, but Land Rovers can get kinda complicated when it comes to the factory sound system and the built in amps and stuff like that. but thats just my .02.
good luck!
