What @Gswffye says is the main issue with your circuit, Vs should be connected between the two FETs.
There is an app note by Infineon I came across (unfortunately can't find it at the moment) about sizing of the bootstrap capacitor and perhaps more importantly the properties of D1 in your circuit. Most notably the diode should have a fast recovery time (sub 100ns ideally) to prevent transients from the switch node flowing back onto the supply rail (particularly important when the FET supply is greater than Vcc).
It may also be appropriate to add a diode across the series gate resistors to aid in turning off the FETs quickly. Under your current schematic the gate capacitance of the FET has to discharge through the resistor which slows it down and results in the FET staying in the Ohmic region for longer. This may result in the FET blowing as it heats up.
With regards to the gate resistors consult the datasheet of the IR2104 and see what it's drive capabilities are, you want the resistor to be large enough to limit current to protect the output circuits but ideally as small as possible so that the gate capacitor charges up as quickly as possible resulting in the FET turning on as quickly as possible. The pull down should be between the gate and source. The size of this doesn't really matter too much, tens of k or higher are fine - it's there primarily to ensure that the gate doesn't float (particularly during start up). Remember you're in effect making a potential divider here between the output pin of the driver, the gate and the source, too low a value will result in not a high enough Vgs to turn on.
The point about transients isn't to be taken lightly - I recently built a ~1kW 340V H bridge driver for a motor using the IR2104. Without the additional protection (gate diodes, pull-downs and fast recovery diodes) the driver and FETs would die.
