You've been a member for almost 8 years. Do you not already have a bike that fits? If so, use that as a guide. 51cm effective top tube on a road bike sounds about right for someone 5'7" unless you are oddly proportioned. Publishing stack and reach figures is a relatively new convention. Ebay sellers of vintage bikes are not likely to know those numbers. I'd suggest you sort out whether you want a bike to ride or just enjoy noodling with geometry. Both types of folks here on BF.
