How much tank you can afford would be a good place to start. I'm of the opinion that bigger is better but recently learned that it's exponentially more expensive also. I have a 55 and wanted more fish, so I got a 125. the 125 is going to cost 6x what the 55 did. But my 55 had HOB filters and the 125 will have an underneath sump/fuge filtration system, a lot more expensive but potentially offers the fish "the best possible living space".