that makes sense, and having to use a ruler sounds horrable!
well I was just thinking that since 1/6 and 1/12 is the most common scales (play scale and dolls houses) so therefor there would be a lot of easy accessable probs etc.
but i guess you have to make most from scratch anyway, it just sounds like a lot of extra work (I'm thinking about making kind of a longer film this time, so there will be a need of A LOT of probs and sets)

I'm more of a fat-fingered oaf than Kelvin, I had to try sculpting heads at a totally realistic 1:6 scale instead of my usual slightly oversize heads, and really struggled with the small features.  Gave up on the child heads - the size of the tip of my thumb -  and passed the job onto someone else.  1:12? No thanks!   I have some puppets that are only 5 or 6 inches tall, but they are like children or  wee gnomes with much bigger heads and features, so they really feel like a bigger scale.  Their noses, mouths, and ears are actually much bigger than a realistic 12" tall human's would be, so they can be animated despite the 6" height.  A 6" realistic adult human is another story.
Also a good point about the camera - the smaller you go the harder it is to get a decent shot.
And you have to scale the moves down in proportion with the puppet size - a 6" tall puppet taking 12 frames per step needs to move half as much per frame as a 12" puppet also walking at 12 frames per step.  Slow moves requiring very small moves become impossible.
If you have a very stylised look, like Nick Park, Adam Elliot or Tim Burton, you won't find anything ready-made that will fit in your universe - you'll have to make it all anyway so it has your distinct look.   The height of a puppet may suggest one scale, the size of their head and hands another.   If a puppet with very short legs needs to sit in a chair, it needs very short legs too, a real-looking one just won't work.
If it's closer to real, but set in medieval times where everything is hand-made, the same thing goes, you'll make everything anyway.  So go with any size that feels comfortable.
It's when you want  it close to realistic, and there are things from the modern world that can be readily bought, that basing it on a set scale is useful.  I started with approximately 1:5 scale, but went down to 1:6 so I could use some ready-made 1:6 scale motorbikes.  They are fiddly with a lot of exposed detail so it was worthwhile.  But they wouldn't look right for Wallace and Gromit, their motorbike with sidecar had different proportions that suited that world.   
I also use 1:24 as a smaller scale for really wide shots that I couldn't fit in my studio at 1:6, like a whole street, because I could get plenty of 1:24 cars fairly cheap. It's too small for character animation but can work as a second scale.  I don't attempt any detail on my 3" tall mini-puppets, they will never be seen up close.  I cut back to the 1:6 set when I want to see some acting.
With the digital compositing now possible on our home computers, I could possibly animate the puppets and enough set to put behind them at a large scale, and blend that with the wider view shot at the smaller scale.
There is a lot being made for 1:6  action figures these days, like clothing and small props.   So even though my puppet heads and hands are a bit bigger, it helps if they can wear 1:6 clothes that I can buy from China on eBay.  Saves me a lot of sewing. 
I have to make the hats though.