Hi, I am new here and am sure other members have their preferred way to resize.
What I used recently is picnik.com - it's free
Go on their site, click on Get Started Now
Browse through your photos and choose one
Then on their webpage they have a heading "resize"
The attached photo was 2640 x 1980 so I typed
in the boxes 600 x 450 and this is what I got
Then you can save the photo with a new title
One thing I have learned over the years is to make a copy of my favourite photos before playing around with them, as I have made a mistake and lost them sometimes.
ps If you preview post you can see the size your photo is going to be
Hope this helps