As @mauro3 pointed out, that is not possible.
But you are able to enforce that by defining a inner constructor:

julia> struct A{B,N}
field::NTuple{N,Int}
function A{B}(x) where {B}
N = B÷64
N == length(x) || throw(ArgumentError("Tuple of incorrect length"))
return new{B,N}(x)
end
end
julia> x = (1,2)
(1, 2)
julia> y = A{128}(x)
A{128,2}((1, 2))
julia> z = A{64}(x)
ERROR: ArgumentError: Tuple of incorrect length
Stacktrace:
[1] A{64,N} where N(::Tuple{Int64,Int64}) at ./REPL[2]:5
julia> z = A{64}((1,))
A{64,1}((1,))