Box Sizes

Anonymous - OK, first, you will need to create this column in BoxTable:

Volume = [Height]*[Length]*[Width]

Then you need to change your column formula to this in ITEM_MASTER (you do have some items that don't fit)

Box Size = 
    VAR __Height = ITEM_MASTER[UOM_HEIGHT]
    VAR __Width = ITEM_MASTER[UOM_WIDTH]
    VAR __Length = ITEM_MASTER[UOM_LENGTH]
    VAR __UserMeasure1 = MINX( {__Height,__Width,__Length},[Value])
    VAR __UserMeasure2 = IF(__Height = __Width && __Height = __Length,__Height,MINX( EXCEPT({__Height,__Width,__Length},{__UserMeasure1}),[Value]))
    VAR __UserMeasure3 = MAXX( {__Height,__Width,__Length},[Value])
    VAR __BoxTable =
        ADDCOLUMNS(
        ADDCOLUMNS(
            ADDCOLUMNS(
            'BoxTable',
            "__Measure1",MINX( {[Height],[Width],[Length]},[Value])
            ),
            "__Measure2",IF([Height] = [Width] && [Height] = [Length],[Height],MINX( EXCEPT({[Height],[Width],[Length]},{[__Measure1]}),[Value]))
        ),
        "__Measure3",MAXX( {[Height],[Width],[Length]},[Value])
        )
    VAR __Table1 = 
            ADDCOLUMNS(
            __BoxTable,
            "Fit1",IF(__UserMeasure1<=[__Measure1],1,0),
            "Fit2",IF(__UserMeasure2<=[__Measure2],1,0),
            "Fit3",IF(__UserMeasure3<=[__Measure3],1,0)
        )
    VAR __Table = 
        FILTER(
            __Table1,
            [Fit1]=1 && [Fit2]=1 && [Fit3]=1
        )
    VAR __MinBoxVolume = MINX(__Table,[Volume])
    RETURN
        MAXX(FILTER(__Table,[Volume]=__MinBoxVolume),[Size])

Part of the issue is you were using the measure form in a column form, but makes sense since you couldn't open the PBIX to see the column form.