http://www.redken.com/whats-hot/argan-6/

This is what springs to mind, and a search seems to produce lots of results. Also for ten pence coin/piece if you feel that a hazelnut is a little bigger, although I think five is about right. Obviously this would only work if the intended audience is British English. HTH!

Now I know that a hazelnut is not the same size as a walnut, but I think that 2 or 3 hazelnuts might just equate to this amount. Walnuts are often used as a measure in hair products, and certainly sounds more inviting than a small blob!

http://jamesbrownlondon.com/james-brown-hair-styling-range
Apply a walnut sized amount to towel dried hair running fingers from root to tip. Blow dry as normal.

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Note added at 8 mins (2012-01-27 12:05:24 GMT)
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http://beaut.ie/blog/2009/beautie-investigates-just-what-is-...
I couldn’t stop thinking about walnut-sized blobs of hair product. Would that be just-fallen-off-the-tree walnut now, I wondered? A mature walnut? A walnut sans its hard covering? I JUST DIDN’T KNOW! There’ve been times I’ve woken up in the night, pondering this extremely First World Problem.

So, like the proactive miss I am, I decided to do something about it. I contacted the Kerastase press office, to seek confirmation of my query. No doubt they pissed themselves laughing in the office, but they very nicely responded politely and solved my dilemma.

The answer is …

*drumroll*

You should be using a blob of product the size of a shelled walnut. I’m guesstimating that’s about two centimeters long and perhaps one and a half high?

if it's thick, I'd go with dab, not drop

exact translation will depend on your exact sentence but size-wise it's along these lines.

in colloquial language I'd call this a 'dollop' of something, but that doesn't really fit here.

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Note added at 23 mins (2012-01-27 12:20:50 GMT)
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In French "noisette" is often used for the sort of product that expands when it comes out of its container.

"MOUSSE: A product for all hair types. If you have fine hair, consider using the size of a golf ball, if you have thicker hair, consider doubling that size."
http://barrington-il.patch.com/articles/how-to-use-hair-prod...

"Then apply a dollop of mousse the size of a golf ball from an inch below your roots all the way to the ends."
http://www.goodhousekeeping.com/beauty/makeup/summer-hair

"Spray a quantity the size of a golf ball into the palm of your hand to start with, and then use more if you need it."
http://www.ehow.com/facts_5263033_hair-mousse.html#ixzz1kf0v...

That's what is sounds like to me.

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Note added at 33 mins (2012-01-27 12:30:52 GMT)
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Oops, I see now in Discussion entries that Catharine mentioned this and Tony thinks the register is wrong (but why? it sounds OK to me).

I think the register is appropriate, and 2 or 3 'noisettes' can easily add up to a tablespoon in my experience.

If kids still play marbles. I haven't noticed any recently, but gave up playing marbles some years back.

I don't see anything wrong with using 'blob', and rather than be pedantic about the exact shape, diameter, volume, etc. (otherwise why not say 5 ml or summat?), I'd just go for generous, meaning 'a bit larger than you might normally imagine a blob to be' — and if the worst comes to the worst, the manufacturer is going to sell more of their product, rather than less!

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Note added at 5 heures (2012-01-27 16:59:25 GMT)
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Perhaps not lasting damage... but you can RUIN a hairstyle, dahrling!

Just thought I'd throw this in the pot - it is in my 'hygiège/beauté' glossary so I must have picked it up somewhere. Tony will probably say most of today's consumers have never seen one but there you are ! It does seem to indicate the right sort of volume though, no?

A possibility here. If you are to squeeze "deux ou trois noisettes" to fill your palm, perhaps you could say something like "apply a palm-sized amount to your hair". It seems fairly common on beauty sites on the web.

*

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Note added at 9 heures (2012-01-27 21:18:28 GMT)
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e.g. "Now add mayonnaise, a squidge of anchovy paste, some cream of horseradish (just a tablespoon) and chop up a pickled cucumber very finely diced."