iOS7でNSString#sizeWithFontがDeprecated
代わりにsizeWithAttributesが使えるようだ。
CGSize size = [s sizeWithAttributes:@{NSFontAttributeName:[UIFont systemFontOfSize:20.0f]}];
【参考】
http://stackoverflow.com/questions/18897896/replacement-for-deprecated-sizewithfont-in-ios-7