Take your time and think it thru...
Its easy to say go big....
When I was deciding, I thought about a lot of things, Sure it would be awesome to have a 300 gallon. Then I thought about , would my equipment be able to handle it ? What would I need to upgrade ? $$$. Then I thought about dosing, media, all of which I would have to increase...$$$$ All this was in my decision making..But the one thing that helped me in making a decision was the maintenance....The old saying 25% W/C month. That would be about 75 gal a month, man thats a lot of water, time, effort....
So then I decided what I would be comfortable with in doing maintenance. I didn't want to be a slave to my tank. If this was the only thing I had to worry about, then not a problem. But unfortunately I have many more responsibilities. I then decided on a 170 gal. I'm glad I did. Its in my comfort zone. I have no problem doing the maintenance, and keeping it looking its best...