Click on the block you want to resize to select it. There are six buttons on the left of the block. Click the cog-shaped button (at the bottom) for Settings. It opens up a menu to change the width and height.
Piktochart provides pre-set width e.g. 600, 800, and 1280 pixels width. If you would like a customised width, open the drop-down menu under Canvas Width and choose Custom. A textbox to input your desired infographic width should appear. Please note that changing the width on any block will apply the changes to all blocks. You can also input your desired height (in pixels) for the selected block.