Are there any problems with database extents not being uniformly sized?

Are there any problems with database extents not being uniformly sized?
Do database extents need to be uniformly sized in a Storage Area?
Is there any performance or administrative requirement for area extents to be equally sized?
How to resize Storage Area extents?
When a variable extent becomes a fixed extent after a new extent is added with PROSTRCT ADD, it is not the same size as the previous extents.

No, this is purely an aesthetic preference. There's no explicit requirement for Storage Area extents to be equally sized.  Over time, irregular fixed extents usually result when Storage Areas need to be extended.  Refer for example to Article:
 What happens to the variable-length extent when a new extent is added  

One potential issue could be if that an extent is GT 2GB and at some time needs to be accessed in a non-Enterprise database license environment (like a development environment for example).  Refer to Article 

• How do prorest and procopy work with Large File Support?   

Should database extents all need to be equally sized or adjusted in size, the database needs to be either:

1. PROCOPY or PROREST to a new structure with the required extent sizes defined in a new database structure file (.st). Refer to Article:  How to fix Progress database variable file extents?  
2. TABLEMOVE database objects out of the existing area to new area(s), that are added with the required fixed size extents, then remove the previous areas (PROSTRCT remove). An example is provided in Article  How to restructure the database using TABLEMOVE and PROSTRCT REMOVE  
3. Dump and Load to a new structure with the required extent sizes or Dumped, Deleted and Loaded to new area(s) that are added with the required same fixed size extents, then remove the previous areas (PROSTRCT remove).