DAX Measure Assistance

Anonymous 

See if this works:

Min Site size by Group =
MINX (
    SUMMARIZE (
        FILTER (
            ALL ( Sites1[Site ID] ),
            NOT ( ISBLANK ( [Total Reading Grouped by Site Size] ) )
        ),
        Sites1[Site ID],
        "size", [Site Size]
    ),
    [size]
)

Min Site ID by Group =
CALCULATE (
    MAX ( Sites1[Site ID] ),
    FILTER (
        ALLSELECTED ( Sites1[Site ID] ),
        [Min Site size by Group] = [Site Size]
    )
)

Total reading for Min Site ID by Group =
CALCULATE (
    [Total Reading],
    FILTER (
        ALLSELECTED ( Sites1[Site ID] ),
        [Min Site size by Group] = [Site Size]
    )
)

Min Site ID Text =
"Site ID " & [Min Site ID by Group] & " at "
    & FORMAT ( [Min Site size by Group], "Standard" ) & " sqft is the smallest store in this group with a total reading  of "
    & FORMAT ( [Total reading for Min Site ID by Group], "standard" )

I've included the sames calculations to obtain the max site values and text in the attached PBIX file:

Anonymous 

try:

MINX(FILTER (
ALLSELECTED ( Sites1[Site ID] ),
[Min Site size by Group] = [Site Size]
),

[Total reading])