ok you have to consider 2 things.
1) max freq of use
2) type of resistor
#2 will have a significant effect on #1.
Altho lower wattage rating, carbon resistors are normally used so that you dont have a hi inductive reactance problem that would occur if you were using hi wattage wire wound resistors. This inductive reactance problem increases with frequency
You will need multiple resistors to produce a) the wattage capability you need and b) to still end up with a 50 Ohm load.
Have "been there done that" in years gone by as many of my fellow amateurs have.
There are much easier ways these days that werent available a way back then....
there are now 50 Ohm terminating resistors available capable of at least 100W when correctly heatsunk.
here's an example www site showing what they look like
http://www.innovativepp.com/resistors.html?gclid=CKiRoZDv4KcCFQXybwodYQ599A
cheers
Dave