I am still on £1 a bet after 3 moinths, reason being I can't find my style for spreabetting, so as said above, staying at £1 until consistently profitable.
When you progress to bigger stakes (and IF I ever do) you might think about the 1% rule - this is that whatever stop-loss you use it shouild not take away more than 1% of your total trading capital. e.g. if your trading capital is a round £10,000 to make it simpler, you would not want to lose more than £100 on any one position. If you're trading an index and you determine not to let the position go more than 20 points against you, that would suggest your max bet size would be £5, so that your max loss would still be 1% of your £10,000.
1% is a figure commonly used. When you get more consistent, you might venture to up it somewhat, but don't lose sight of the ground, it can be a long fall.