The minimum trade size will depend on the product that you’re trying to trade.
For currency pairs, metals and other commodities, the minimum is 0.01 (often 10c per pip)
For indices, we've worked with our LPs and have been able to offer "micro" lots now on indices. For mini lot (0.10) you can trade 10c per point (rather than the usual $1 per point on most indices).
To do this, just enter "0.01" or above in the MT4 Volume ticket. It's a great way to dip your toes into these markets.
Please be aware that different products have different notional values. We advise you to place trades on a demo account to familiarise yourself with these different trade sizes.