A good rule of thumb for either a mini-account or standard forex account, is to limit your margin usage for each trade to 5% – 10% of your usable margin.
As an example, if your usable margin is $5000, to trade safely, limit your margin usage for each trade to a maximum of $250. This means trading only 1 full lot for each trade. This is assuming that you are trading in a CMS Universal account with 400:1 margin. Your use of margin is increased with a smaller ratio, as most other brokerages only offer a smaller ratio, normally 200:1 or even 100:1.
As your account grows and your usable margin grows, you can increase your margin usage and trade bigger mini or full lot sizes. If you lose money and your account shrinks, drop your margin usage back down to smaller sizes. You need to learn to keep your eye on your usable margin, especially if you’ve suffered some losses.
Protect your usable Margin by not having more than 2 open hedged or unhedged position at any one time. Your usable margin & equity will get eaten up by un-hedged open positions that go bad in the wrong direction…this is a really good reason why you want to use stops, and if you hedge, hedge tightly.