Spark asks for 3% with a 6% static floor and a 2% maximum loss on any single trade. That last rule shapes everything.
Work backwards from the stop
On a $100,000 account, 2% is $2,000. If your stop on EUR/USD is 25 pips, one standard lot risks about $250, so the rule caps you near 8 lots — but you should not go near that. Risking 0.5% ($500) per idea means four losers in a row cost 2%, leaving two thirds of the floor intact.
Use the ticket
In the FQ Terminal set the stop first, type your risk in percent, and the ticket computes the lots. The preview shows the dollar risk before you send.