Thank you NZScion and tdracer for your comments, you have put me on the right track to research more on the issue. So the fan diameter is not the "culprit" in the windmill relight so much as the by-pass ratio is.
I've found confirmation on that looking at the in-flight restart envelop charts: MD82 (Low by-pass engines JT8D-217) 180kts or more, A320, B738 (Hi by-pass CFM56) respectively 260kts+ 300kts+ (talking about pure windmill relight).
B_D