You can override that by:

1) Modifying the data structure of the blogs table: the "description" field is a varchar(255). You can change that to TEXT for example. PhpMyAdmin is an easy way to play with your DB structure if you're not too keen on MySql command lines

2) Then, In Settings -> Blog settings, make sure you update the "macimum length" value to something more substantial

Note that I haven't actually tried this trick... just an assumption :D