hi please can you tell us how to make the posts bigger so you can put bigger topics

Do you mean a higher number of characters for a post? If so then this can't be changed.

i mean so can put more writing in the post

You mean on the Topic title? if yes it can be edit in admin panel --> General -->topic title length

where is topic bit cant find it

This option can be found in "General / Config" >> Topic Title length. For the lenght of the posts, there is a limit of about 16.000 characters per post and that can't be changed. Any message that extends these limit has to be split in two parts...