Blockwork type and thickness?

I am building a workshop in the carden which will be around 7 metres long x 3.5 metres deep x 2.6 metres tall. It will have a flat roof and the base is 150mm thick reinforced concrete on 150mm well compacted hardcore.
Can anyone help give a clue what thickness and type of block I should be looking to use? I am guessing 150mm thick would be ok, but have no idea what type of block I should use and whether I need to incprporate a pier in the corners or within the 7 metre run, thanks, Alan

 

I am looking to have it rendered in sand and cement. It may be a year or so after I have built it before I have the funds to have it rendered though, so it could well be unprotected from the elements.

 

Thanks, what sort of block would it be if I used 6", would a standard celcon 3.6N/mm2 block be ok?