u need to find the target frequency , gate counts for ur design to select a "FPGA",
and also the COST of the FPGA,
MANY FPGA VENDORS like XILINX,ACTEL, ALTERA etc ,are there,
u have to choose one of them that suits to ur BUGET both in terms of design parameters and also the KIND of environment ur application will be working.
It depends of "how long is the algorith", it means how many logic elements and gates you uses, also it depens the program you want or like tu use, i prefer Altera`s fpga because of their flexibility and powerful program and simulation tool , Quartus.
What makes you think so?!!
I believe that there are many parameters with orders,
A-COST
B-SIZE
C-SPEED
they are not independent parameters, but if you have a predifind cost then you will be limited with speed/size. on the other hand if you have a specific speed requirement this will affect your size and cost. However, usually you have a fixed budget then you go through the comprimising process between speed/area. other parameters may exist like power consumption, temprature of environment the design can handle ... etc. usually these parameters will affect the cost effectively.
Blindly you start ur design entry with the target device set to any FPGA, once you finsih the synthesis, you will be in a position to select your device after seeing the synthesis report.