Try using imresize() to scale your image to fit the network dimensions.
Usage Example:
% Access the trained model
net = alexnet
% See details of the architecture
net.Layers
% Read the image to classify
I = imread('peppers.png');
% Adjust size of the image
sz = net.Layers(1).InputSize
I = I(1:sz(1),1:sz(2),1:sz(3));
% Classify the image using AlexNet
label = classify(net, I)
% Show the image and the classification results
figure
imshow(I)
text(10,20,char(label),'Color','white')
To learn more about Transfer Learning, refer https://www.mathworks.com/help/deeplearning/examples/transfer-learning-using-alexnet.html
