To size your canvas specifically to various social media types, you can do this two ways in SampleBoard:
Start your moodboard with a pre-sized template created for social media
Start your design with a custom canvas, then enter the dimensions you want from the handy chart below using the canvas settings tab within the Creator

Canvas settings panel
SampleBoard’s canvas settings can be found in the Right-Side Palette and enables you to change the size (dimensions) of your canvas by allowing you to adjust the width and/or height to suit your moodboard.

You can adjust the canvas size at any time during your creation. So for instance, if you started with a landscape orientation, but then decide portrait or square would be a better choice, you can easily change the canvas width and height by changing the pixels.
To make things a little easier for you, here’s the dimensions in pixels for common Instagram and Pinterest sizes to promote your moodboards on social media for best results.
Common social media image sizes (in pixels)
Instagram Post - 1080 x 1080 px (1:1 ratio)
Instagram Portrait - 1080 x 1350 (4:5 ratio)
Instagram Story - 1080 x 1920 px (9:16 ratio)
Pinterest Square Pin - 1000 x 1000 px
Pinterest Standard Pin - 1000 x 1500 px (2:3 ratio)
Facebook Post - 1200 x 1200 px
LinkedIn Post - 1400 x 800 px
How to instantly resize a canvas
Click the canvas settings icon on the right-hand side toolbar
Select 'fixed' layout
Enter the width and height in pixels
Your canvas will instantly change size

Once your moodboard design is complete, you can:
Download it to your computer and upload onto social media
Share it via email
Set it public and share a link on any social media platform