Media Toolbox can resize a partition, but the effect of that resize depends on the file system installed in that partition. We don't have any AmigaOS file systems that can suffer a partition resize without losing data.
Since the file system could have data reaching to the extremes of the partition (start and end), any resizing that loses "edge" data could easily be fatal. All Amiga file systems use the first block of the partition to store the "signature" of the file system, so you can't chop off the start of the partition. Likewise, depending on how full the partition, there could be data out to the end of the partition, and losing that might screw part or the whole of the partition.
If the partition is expanded, then the increased space can only be used if the filesystem is made aware of the change and is capable of recalculating its geometry to make use of it. That usually requires a Format to generate the new data structures.
It's safe to assume that any reduction in the partition size will kill the contents. An expansion probably won't hurt, but won't be useful in any AmigaOS file systems.