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First: I am sorry to say, but your question is too generic, and "simplified." Therefore nobody can answer it in real terms.
I would present the question in opposite way:
"I need this much logic. Which FPGA is big enough, i.e. has enough logic resources, registers, and IOs for my needs? Logic level compatibility on IOs? Speed? Price? Packages available?" About speed: You can NEVER achieve( the theoretical limit if you hav any level of complexity in your design. (Who wants only a simple and short binary counter and a few multiplexers in a FPGA!)
Then you can assume that the FPGA should have quite much more resources than you think you need, because there are always some issues such as:
* The logic complier does not generate everything that optimally anyway
* The "fitting" process (placement and routing) has it's limitations: Amount of routing channels and "smartness" of the software.
If you want to design in a HDL it is better to stick to a standard language (VHDL, Verilog), if you do not want to bind youself to one supplier. In the other hand, supplier specific tools may give you better control of the result -- languages such as AHDL for Altera have better "manual optimization" possibilities compared with for example VHDL.
I would start from the requirements and work a bit later with the selection of the technology when you have a good picture what you really need.