Yahoo has posted that as of now, all Yahoo Search Marketing ad descriptions that are longer than 70 characters will be truncated to fit the new size requirement.
There still is an optional 190-character long description box, which may be used on some of Yahoo’s distributing partner sites. So if you have not manually revised your ads to the 70-character limit, you may want to review your ads now.
Source:→ SEL Blog
Yahoo!, Search, Search Marketing, Search Ads