Hi,
I have added a link for you to a Dex210 gear ratio chart
Team Durango Gear Ratio Chart
Like already suggested if you are looking for top speed you would generally go for a larger pinion or if you are wanting more bottom end then you might go for a smaller pinion.
Always check when changing pinions that your motor is not getting to hot after your run :-)
I currently run a 8.5t motor with a 25 pinion on 87 spur