Much depends on a few important factors:
1) How much are you planning to be involved with feedback and direction to Designers?
Asking Designers to "fly blind" will likely result in less entries, and fewer higher skilled designers.
2) What are you giving to designers to start with?
Your brief and sliders are important. You will get what you give. Give little, get little. Give more, get more. Reference material is a launching point for designers.
3) Prize
A larger prize will usually translate into more skilled designers feeling your contest is worth the effort. Remember, designers are effectively risking working for nothing unless they end up your favorite design. Designers are not selfish but they know their worth and consider a number of factors before entering. Also to be considered is what are you asking from Designers compared to what you are willing to pay for it. If your prize does not reflect a fair balance then you should expect a lower amount of entries.
You may pay a $50 premium to have your brief information sent to all designers on the site who have the notifications checked in their portfolio.
Some clients do that while some prefer to funnel that money into increasing the prize.