This is the only thing that is stopping me using my spreadsheet, but I believe it is so important to encourage new traders to not blow their accounts, that it should be included by default on myfxbook.
Somewhere on the main screen per account, that shows the recommended lot size for the next trade. It works this out by using a stop loss size field (editable by the user but remembers the previous value), a preferred risk % (default to 2% but editable by the user and remembers the previous value), the account balance (capital) and any open trades (SL in pips * lot size).
For example, if there are no open trades, and SL is set to 50, risk % is set to 2% and capital is $1,000, then the recommended lot size for the next trade would be (1000*0.02) / 50 = max risk of $0.40 per pip, which divided by 10 equals our volume/lot size of 0.04.
If there are any open trades it should take them into account as well, assuming the % risk is the maximum the user wants at any one time i.e. all open trades should equal a maximum loss of 2% if they all go wrong.
I think this would encourage better money management from newbies who with the leverage offered are at risk of blowing accounts without it.