Definitely say $20 and $100 contracts are much better for beginning.
$1000 will be to risky.
Probably $100 would be more realistic, but we'll see from the explanation below. Also as time goes, if the load to the exchange is too big, it could always be changed.
How much leverage do you plan to use for marginal trading?
1:3 or 1:10 or ?
This is very interesting part. The maintenance margin requirement is a variable value, the way it's calculated will always be public to prevent abuse of course.
On traditional exchanges, this value is between 2% - 10% of the asset's current market value. From a real world example to buy 1 lot of USD vs. somecurrency contract for $1000, one would need $50 maintenance margin.
One more question -- from your contract spec. :
"Minimal price step: 1
Cost of the minimal price step: 1"
What means "1" here - 1 BTC, 1 USD or 1 magic point ?
I was not very clear in this. Let's look at the formula:
VariationMargin = (LastPrice - PreviousPrice) * W / R;
where W is the cost of the minimal price step and R is the minimal price step. LastPrice is the current price, and PreviousPrice is the price of the previous "clearance" or just a start price if the contract has just started trading.
In order to better understand, let's make a real example, what today's highs might look like (for simplicity, assuming futures trades without contango or backwordation) along with all calculations.
24 hours ago, 1 BTC max price was $2.955 USD, out of this, $1000 USD corresponds to 338.4 BTC.
1 hour ago, 1 BTC max price was $2.86 USD, out of this, $1000 USD corresponds to 349.6 BTC.
Hence, the necessary trading precision for $1000 contract could be 1 BTC (trade $1000 contracts with roughly 2.86 USD step in the price), or 0.1 BTC for a finer grain (ca 0.286 USD price change for the today's ratio).
Please note, it might be hard to understand at first, but it looks different than just spot rates. See - the BitCoin's worth fell today from $2.95 to $2.86, but the corresponding futures contract value increased!
So if you want to profit from BitCoin becoming cheaper you need to long-buy the contract, and if you think BitCoin's value is going to increase, you need to short-sell the futures.
I'm used to how exchanges work, but if you think there is a mistake somewhere or you can tell how to make it better - please post here.
. We could introduce capped futures to cope with this, or charge a special small "insurance" value which would be later used to refund losses in these cases to contract counterparties. 
