Hi there 

Thanks for posting in the community! The simplest way to center your diagram on the canvas is to press Ctrl + A to select all shapes then manually click and drag to position the diagram to your liking. Additionally you can adjust the size of your canvas to better fit your smaller diagram by navigating to the Page Settings panel on the right-hand side of your screen toggling the Auto Tiling option to the off position then clicking on your canvas. You will see three tabs with arrows appear - one on the top right one on the bottom left and one on the bottom right corner. You can click on these tabs and drag to resize your canvas appropriately for your diagram.

Lastly you may find the Grid and Rulers options helpful for ensure precise adjustments via the methods above. You can enable both of these views in the View menu within your Lucidchart document. Hope this helps! Please let me know if you have any additional questions.