The most common number of characters available in the short description seems to be 40.
The long description, often called the purchase order description, should be the the full version of the short description. It is not a continuation of the short description.
The long description is the long, unabbreviated version of the short description
In SAP, the long description is normally 72 characters wide, so it fits well on the printed purchase order, and it has, in theory, no limit to the number of lines. However, if the long description ends up longer than 20 lines, then there is probably something wrong with the way the line is being created.
Regardless of the tool / system / ERP / maintenance management software / database you will probably have many field length limitations to work with.
This is not all bad as it keeps many consultants gainfully employed!
Working succesfully within the limited space requires discipline. If rules are not strictly adhered to, data gets entered in a free text format and there is no consistency from one record to the next. The number of duplicate records multiply with alarming speed.
Structuring a description is not particularly hard or challenging. The best tools construct descriptions dynamically. This gives the control required and allows multiple descriptions to be derived.
Some of the considerations to consider:
What CASE is the description?
What separators are going to be used?
What abbreviations are going to be followed?
What is allowable in a description?
What truncation indicator is going to be used?