Brad Lawryk

In the Transform Panel, type sw (spread width) in the Width box and sh (spread height) in the Height box.
1. You must have the aspect ratio link icon selected
2. sw and sh are case-sensitive (Grrrr)
3. Your image may end up off the canvas, if so you can just drag it back on or enter 0 and 0 for the x,y coordinates in the same Transform panel
4. If you do this a lot, just get used to immediately typing 0,0,sw,sh whilst tabbing between the fields
5. The above will make the image closely match the document size, but it won't be an exact fit due to the different aspect ratios of different images

Expressions for field input  

@Brad Lawryk  I have found a very easy way, thanks to Dave Straker @dmstraker to do this.  It may be a wee bit fiddly, but for me it works perfectly.  He posted this back on June 29, in response to the topic: Realistic Wet-in-Wet Watercolor for Affinity Photo:
"When starting from a stock photo, try:
File/New then just hit 'Create'. Doesn't matter what shape/size the canvas is. Find and drag in image from Stock tab. Document/Clip Canvas. Layer/Rasterise. "The Clip Canvas will automatically resize the canvas to fit the image."
In order to get the image into the right size, I start with the blank new document.  Then after his Clip Canvas step (3), I don't rasterize the image layer.  Instead, I use the Move Tool to reduce the size of the image to something much smaller.  Then I Copy that image layer, and move to the document in which I wish to place the image, and hit Paste.  The image can be resized any way you like in the destination document.  (Also rasterized there.)
 

Hello Community,
I am an Adobe Photoshop convert and so far I love Affinity Photo! I have been able to figure almost everything out from my usual PS tasks.
I have one question about using Stock Photos. I really love how I can simply drag an image to my comp. Amazing.
However when I drag an image into my comp it is very large compared to my composition. It takes forever to scroll around and find the resize handles. Is there a way to drag an image in and have it constrained to document size?
Loving Affinity. Wish I had discovered it sooner.
Thanks in advance.

Thank you so much for the quick reply! 
The scale method works well. I didn't know that was an option. Thanks for your screenshot!
 

Thanks Dan!
It was indeed the 'Booklet' option that I needed. All is good now.
I am enjoying my transition from Adobe to Affinity so far.
Thanks again!