"<signal> is not a constant" error in if-statement

I am trying to write a simple module to output a 14-bit number based on the value of four input signals. My attempt is shown below.

module select_size(
    input a,
    input b,
    input c,
    input d,
    output [13:0] size
);

    if (a) begin
        assign size = 14'h2222;
    end
    else begin
        if (b) begin
            assign size = 14'h1111;
        end
        else begin
            if (c) begin
                assign size = 14'h0777;
            end
            else begin
                assign size = 14'h0333;
            end
        end
    end

endmodule

Upon compilation, I receive the following error:

ERROR:HDLCompiler:44 - Line 67: c is not a constant

I don't understand why that particular if-statement isn't working if the other two preceding it are. I have tried changing the condition to

if (c == 1) begin

but to no avail.

Does anybody know how to solve this error? Thank you!