there are some default attributes in reviews and rating section, they are Product,Posted By,Summary Rating,Detailed Rating,Status,Nickname,Summary of Review and Review. now i want to add one more attibute named "selected size", how can i finish that ?
Reviews are not EAV entities. So extending this is somewhat easier.
1) You can add a new field in the review_detail table and extend the module to also save this new field. You will not need to do too much work at the module level (mostly the effort is to display this new field in new review form and related views).
2) You can create a new table (don't alter Magento's core db Tables) with a reference to Review (review_id) and your custom field. This also means to extend Review model's and use afterSave, afterLoad functions to save/retrieve your data...
3) Duplicate entire module logic with custom tables (I've seem some more than a few commercial extensions that have this approach to do stuff with reviews, inventory and so on). Not recommended.