This has to do with the table regions in your report rdl.
SSRS in the backend converts all table regions to a vb code.
this generated vb code has a size limitation of 64k.
It occurs when you have wide table regions.
which actually contain n*m textboxes and which have to get converted to vb code as textboxes.
There can be two ways to try and remove this error:
either reduce the no of columns in the wide table.
if business demands the columns, then we could try to split the tables into two and make it look like one.
Since the vb code generated is per table region, this reduces the size of each vb code per table.
Another simpler thing to try before splitting the tables,
is to try and renaming all textboxes, controls in the table to a smaller name.
try something like t1, t2, t3 ...
this would also reduce the size of the vb code to some extent