Confused with type definition

New to Julia
9

not sure why it’s not working in the REPL tho

julia> struct MyVec <: AbstractVector{Float64}
         v::Vector{Float64}
       end

julia> MyVec([0.0, 0.0, 0.0])
Error showing value of type MyVec:
ERROR: MethodError: no method matching size(::MyVec)
Closest candidates are:
  size(::AbstractArray{T,N}, ::Any) where {T, N} at abstractarray.jl:38
  size(::Base.AsyncGenerator) at asyncmap.jl:409
  size(::Core.Compiler.StmtRange) at show.jl:1874
  ...