SPD Workflow string length

There is no function to work directly in SharePoint designer workflow for trimming the text or string.
You need to tweak the some logic to achieve this.
First create the calculated column (To find the length of particular column )
In formula paste this code

"=IF(LEN(Description) >10,"OK","No")"
So your calculated column will result either OK or No,
Compare this value in the SPD Workflow, as your requirement use the "Action"
"Extract Sub string from start of the string" and store the trimmed text in the workflow variable and update the item.

And the workflow result on the Item.