My standerd blocks have become very small when I insert them

Whatever drawing you took them from probably had a different scale and or parameters and now they do not match the current scale. You would have to change the scale of the blocks in the drawing they were created in to match the current scale to want use. Then bring them over into the new drawing.  

Were your standard blocks drawn at full size (1:1 scale)?  Yes or No.

Nivek73, are the DDUNITS settings equal on the 2 files, the one in wich you want to insert the file/block and the one in the block/file inserted ?
If they are it should mean that the 2 were not drawn at scale 1/1 like John said.
If they are not it should mean that :
-1- The 2 were not drawn at scale 1/1 or
-2- there is at least one file file with NO insertion scale declared

Yes thy work fine at first

Take a look at this...http://www.cadforum.cz/cadforum_en/qaID.asp?tip=4581