I want to know if anyone has purchase the 11 inch air

Yes... why?

I use 13 inch MacBooks and I think they're the finest products Apple has ever made. There is no functional difference between the two sizes, although the 13 inch model will have slightly greater battery life due to its slightly larger battery. The smaller model is extremely compact, and as such is ideal for someone whose needs value portability among other factors. The size of its screen is a bit too limiting for me but that is a personal decision only you can make.

Their durability is also impressive, having tolerated the abuse of young school children, for whom I oversaw the purchase of a few thousand for both public and private educational institutions. My initial concerns of significant breakage incidents have long since been allayed. I have used them personally (as well as other portable Macs going back 15 years) in extensive travel, and they routinely withstand that abuse also.

I have a 15 inch macbook pro and its 5 years old and getting very slow and unresponsive

The reason for that can probably be addressed by posting a new question. Age itself does not cause a Mac to become slow and a five year old Mac is hardly obsolete. What you do with it during its life may place greater demands on it though. There are many potential causes for poor performance, most of them user-inflicted, and easily fixed for no money. More memory is always beneficial, and can be added for little money. In any event some troubleshooting information would be required so if you are interested please post a new question.

The only notable exception to that might be the mid-2010 model MacBookPro6,2 which was notorious for GPU (graphics related) failures. They were subject to a recall program but it ended recently, leaving them prohibitively expensive to fix. The manifestation of that problem was display-related though and poor performance itself was not a distinguishing characteristic.