using string functions on plpgsql

I'm starting to create my firsts plpgsql functions, and I was
wondering how to use the string function 'length' in the code of the
fuction. I have tried so far and I get an error. I'm trying this:

declare
i integer;
begin
...

i = length(texto)

...

where texto is a varchar(200) I pass as a parameter to the function calling.

Any help will help.

Thanks!!!