The acreage of Sahara Desert is 2,239,998,000 which is 3.5 million square miles. Yes it was once fertile. Watermelons might have even been grown there.
You can argue the size of it. The Encyclopedia Britannica says the Sahara is 3.32 million square miles. The Live Science article claims it is 3.8 million square miles.
Acreage of Sahara Desert William Lawrence Sr Omnibit 5-23-20
Wow! Climate change is amazing! The Greenies would have gone bananas (or maybe watermelons?) then.