You are using Exclaimer Cloud and creating your signature template. You have to select the font size within your signature template.
You set a value of 13, which you believe to be in Points (pt), but when testing and reviewing the email, you see that the font size has been set to 10pt instead.
The most likely cause of this issue is that the font-size value would have been set at a pixel value.
Ensure that the value you are setting is exactly as required, and double-check that the value is set to pt rather than px.
In Exclaimer Cloud, you can set pt or px and swap between the two within the Font Size drop-down list.
Alternatively, you can set the value as px instead.
For more information, please refer to this guide for a Point to Pixel conversion chart: Pixel to Point conversions.