I always try and use a certain lot size depending on my account size and not deviate...Its how you can really help limit your downside...For instance, if you have 1000$ account, use 1.00 pips at the most. IMO, it wouldn't be smart to use say, dollar pips on anything under 500$, the risk to your account on a bad trade or market flucuation is too great for a single trade...
For me, the max I do on the account size is like this...
1.00 pip - 1000
5.00 pip - 5000
10.00pip - 10,000
I may be being a bit oversimplified here, but helps me with risk...