Ah! you have fallen into the trap, the amounts can be quite different between varieties.
For instance earlies tend to be smaller so you get more to the pound, 2nd earlies can be a bit of a mixed bag, for instance Kestrel can be quite large whereas some of the newer varieties are smaller.
I am lucky in so far as I can have the number of tubers I require counted out for me then they are weighed.so size does not matter with me.
Having said that if you work on 6 average tubers to the pound (roughly 15 to the kilo) you should be OK.
If you know a variety comes quite small such as Fir Apple you can order less by weight and if you are growing Kestrel order more by weight.
So when buying the 'nets' have a look at the tuber size and adjust accordingly.
OK I know if buying mail order this is out of the question so in you are in the hands of your supplier.
Regarding your other question i.e. space to chit!
I would select the smaller ones 'hen egg size' to chit this way you will get more in the space you have.
Regarding which end ? the following extract from my website might help you in deciding, but don't worry too much if you get the odd one wrong you can always put it the right way up once it has started to chit
Ideally select tubers the size of hen / duck eggs, and set them side-by-side and ‘rose-end’ uppermost, in shallow trays, or old egg boxes.
n.b. The ‘rose end’ produces most shoots, or 'eyes', and on occasions is difficult to recognize.
If this is the case, look for the point where the tuber was once attached to the root system, this will indicate that the ‘rose end’ is at the other end of the tuber.
