Oh, bummer.
Ok, let's wing it. Let's try 600pixels high.
Just to check - do you understand that all web images are measured in pixels, not inches or centimetres or anything else?
So, you'd open your PSD, flatten the layers, then go to the Image Size dialog. You make sure the "Resample Image" box is checked, this is really important.
Then at the top of that window you'll see the height in pixels. Change that to 600, and press ok. You'll see the image shrink (a lot) on your screen.
Let me know once you've done that.