If you're like me you will always want to go bigger!
Started with a 40g, moved to a 75g then to a 150g and now at 96x30x24 300g. My current dream tank is 120x48x36 and IF I ever move again I will make sure I have a large space that can be built around a tank of that dimension.
I went with larger tanks to hold more fish and to have schools of tangs. My current 300g allows me to have a school of yellow tangs and house my favorite fish the powder blue. I originally wanted the 300 to also house an Achilles but I changed my mind as I read more about them and would rather not kill one due to the difficulty. I now want a sohal tang but will need to go larger than my 300 hence my desire for the 900!