The error message is very clear. Look at the first layer in AlexNet - the input layer. It accepts inputs of size 227x227x3. You need to resize your input image:
inputImg = imread('location_of_your_image');
resizedImg = imresize(inputImg, [227 227]);
This should solve the issue.
P.S. I agree with what John mentioned. Please follow his advice so that it's easier for us from next time on.