According to a FAQ on Picasa's support site, the photo-sharing service provides up to 1 GB of free storage for photos and videos to its users. But since Google+ actively encourages storing and sharing photos - its Android app even offers an "instant upload" option - there may have been some concern about storage limits among Picasa users.
As it turns out, there's nothing to worry about. If you're signed up for Google+, photos up to 2048x2048 pixels and videos up to 15 minutes long won't count towards this free storage limit. And Google will automatically resize photos for you when you upload them to Google+, so they stay under the free size limit.
That means only photos uploaded directly to Picasa Web Albums over the 2048x2048 size will count towards the 1 GB of free storage, explains Google. And when that limit is reached, photos will be automatically resized.
Meanwhile, for non-Google+ users, there are slightly stricter rules: photos up to 800x800 and videos up to 15 minutes won't count towards free storage. Again, when the 1 GB limit is reached, larger photos will be resized down.
Simply put, this means that whether you're on Google+ or not, Picasa offers unlimited free storage for photos and videos (under 15 minutes). The difference is that Google+ users can upload higher resolution photos to Plus/Picasa without being penalized.