Boppa
I doubt that anyone will really be specific. There is absolutely no such thing as a standard length - you make them as SHORT as you possibly can because French heads are liable to bend if you push them too hard - when that happens the cutter can be released (or simply snap off) with alarming ferocity (in other words the cutter becomes one or even two pieces of shrapnel) - this is why they've been banned in commercial shops for many years, as even with guarding THEY ARE POTENTIALLY HAZARDOUS TO USE. They also do not meet the requirements of PUWER98 (Provision of Work Equipment Regs) because they have no kickback limitation (in fact they can also pull work - and your fingers -through as well) which is why it is illegal for manufacturers to sell French spindle tops. This is why no one can tell you much about them! I seem to recall that the ones we used to use were generally only about 40 to 60mm long (in a 20mm diameter cutter head) and no more than about 25mm high. Go more than 50mm projection (even on the thickest cutters) and you're asking for trouble. I used to buy the steel (called French head steel - a low carbon steel) in bars about 24in long from Tyzack's in Sheffield (now long gone). I looked at some old receipts this week (pre-Christmas throw-out) and found that the thicknesses we bought were actually 1/4in and 5/16in (6.5 and 8mm) thick, although 3/16in (5mm) was available (only to be used on very small cutters).
BE EXTREMELY CAREFUL WITH FRENCH CUTTERS. Before making any cut you MUST hog away the majority of the waste from the timber using the table saw or planer. The French cutter is really just a motorised scraper. As such they don't hold an edge for long either - as little as 20ft on some denser timbers, and certainly no more than a couple of hundred feet - and you'd be well advised to burnish the edge each time you use a cutter, it's when they are blunt that they tend to break more easily (they get hot, work harden, become brittle and can snap off). Only ONE edge cuts, the other edge is ALWAYS slightly shorter and is there for balance ONLY. They need to be ground with between 10 and 45 degrees relief angle (horizontal to vertical) at the back to prevent them from burning the work. DO NOT HAND FEED UNLESS YOU HAVE ADEQUATE GUARDING - French cutters can snatch work and pull it through, which includes your fingers. You will therefore need to protect yourself with a decent Shaw guard, not these mickey-mouse springs DIY and Far Eastern machinery makers provide - if you don't know what one is go and look at the HSE web site
http://www.hse.gov.uk/pubns/wis18.pdf
In a previous shop (in the early 1980s) we used to do a lot of short run work with French cutters. They are cheap and were relatively easy to make - but even for a trained wood machinist they are twenty times more hazardous to use than limiter cutters. If you don't know how to grind and balance them or you don't have adequate guarding on your spindle DON'T ATTEMPT TO USE THEM. If you do a lot of short run stuff and need to grind your own cutters I'd seriously consider the Armadillo blocks now on the market, they're a lot safer and they are certified for commercial use.
Finally, I'll be a real nanny and quote you something from the HSE site: "Vertical spindle moulding machines account for around 14% of all injuries caused by woodworking machines. These injuries all too frequently include amputations of one or more fingers. Common causes of accident include the failure to use cheap and simple safety devices such as a false fence, pressure pads or work holder." You have been warned!
Scrit