A tab should be big enough to hold the part down throughout the entire cut but able to break away when pulling the part away from the material sheet.
For a look at the default tab widths, they can be found in the Material step of WAM. Click the add/edit material button and find the material you are referencing. The tab width is found there and will always default to the material setting.
If you would like to manually adjust tab widths for one specific cut file, adjust the tab size in step 5 of WAM.
If you would like to manually adjust tab widths for a specific material setting, copy the reference material setting found in WAM and create a new/custom material with a tab width you prefer - this way it will always default to this width when using the custom material.
Finally, for a quick visualization of tabs along the paths I recommend plotting the gcode file on the website ncviewer. For a guide on visualizing gcode, give this article a read!