How to downsize without losing quality

Hello, I been editing some photos, save them in PSD, the weight is like 5 mg to 8 mg, size like 4320 x2432 or similar. I wan to use this pictures for a web site portfolio and would to keep the same look I got in photoshop with grain effects etc. How can I downsize this whithout affecting quality? I have try to change image size in cs6 or tansform the image , but the quality goes down very clearly. Is there any other way, software etc?

Try with crop tool. Input dimensions in pixels, 300dpi and crop image.

You can try transforming the layer (or layers) in a smart object before resizing.

Select the layers, right click > convert to smart object.
Double click on the smart object layer for open > so you can modify the layers inside.