Position sizes are only determined by money management. It does not matter whether an asset is Forex or CFD. The lot sizes are indeed different, but Zorro normally compensates that.

Run tests for determining the best margin setting for your $3000 capital. I would not disable CFDs, as they contribute substantially to the profit.

FYI, here are the typical asset contributions to the Z12 profit, in percent - look in the "Wgt%" column:

Code:
Portfolio analysis  OptF  ProF  Win/Loss  Wgt%  Cycles
 
AUD/USD avg         .014  1.36  281/475   11.4  XXXXXXXXXX/
EUR/USD avg         .006  1.41   91/115    6.2  XXXXXX\XXXX
GBP/USD avg         .005  1.18   85/105    1.3  XXXXXXX\\XX
GER30 avg           .030  1.33   34/45     2.5  X/X/\.\XXXX
NAS100 avg          .015  1.43  103/182    8.2  XXXX/XXXXXX
SPX500 avg          .077  1.75   96/162   16.4  XXXXXXXXXXX
UK100 avg           .003  1.42   32/32     2.3  .//\\//\\/\
US30 avg            .009  1.30   76/90     6.1  \//XXXXXXXX
USD/CAD avg         .006  1.59   80/73     6.4  XXXXX/XX/\/
USD/CHF avg         .008  1.99   85/97    11.4  X/X\XXXXXX/
USD/JPY avg         .006  1.69  130/175    8.7  XXXXXXXXXX/
USOil avg           .084  1.66  207/286   30.2  XXXXXXXXXXX
XAG/USD avg         .008  1.35  113/152    5.3  \\XXXXXXXX/