Hi @jeffrey k thanks for posting in the community! Unfortunately shapes generated by UML markup cannot be resized. The UML experience was recently improved to fix formatting issues.
 
However we're always looking for ways to improve Lucidchart and your feedback is the best way for us to do that. Would you mind adding any UML feature ideas to our feature request form? Ideas submitted to this form are passed on to the product development team for their consideration.

In the meantime you could resize your UML sequence markup diagram by ungrouping the shapes first. To do this right-click the diagram and select 'Ungroup'. Please note that if you ungroup the shapes you will not be able to use UML markup on those shapes anymore.

Please feel free to let me know if you have any additional questions. Hope this helps!