After trying this in a sample report with multi-value params turned on, I think it could be done with a JOIN(), SPLIT(), TRIM() and then a re - JOIN(), or some such logic, but it will get very stringy. What is the source of your parameter picklist? If a table, you can always create an alternate "short name" that you then pull into the picklist, and avoid the whole stringing exercise, which might degrade performance of the report at runtime, etc. (If it can even be accomplished ...).

Is the data source OLAP or relational? To what are you pointing your report parameter Value and Label fields? You could generate a third field in the supporting dataset, called (for example) Picklist Label, and either place a trimmed field in the table (etc.), or do it as part of the supporting SQL / MDX query, and then reference the new field in the Label field. The tradeoff would be that the name is also trimmed in the picklist - but if you're talking trimming to 10 characters with what you currently have, I'd think that the picklist is overly wide, as it ...

Just an idea ... HTH.

Bill