The general rule in Britain for a normally" powered" man, would be a recommended fixed gear of about 65 in. and for a woman 60in. Obviously crank lengths have an important bearing. A gear of 65" with 6.5" cranks is precisely the same proportion of leverage-to-gear as 70" with 7" cranks. If you live in a hilly area drop it down to 62" or in the flatlands up it to 68". I have been riding fixed wheel since the 60's. If you want to avoid knee problems in later life stick to these ratios. The old timers new a thing or to. That's why people like Titch Morris are still pedaling around on fixed wheel cycles at the age of 92. And there are plenty more like him out and about in the UK. Be very careful and don't go down the macho road "wind milling" huge gears, not unless you want a pair of titanium knee joints to match your Speedwell or Teledyne. The ultimate is a racing tricycle on a fixed or a track tandem on a steeply banked track. Best wishes and be lucky. Michael Butler Huntingdon UK.