Having just replaced struts, I can confirm that they are the same size top/bottom.

Mine were heavily rusted as well, but I opted to only replace the self-locking nuts. The bolt is so big that some light cleaning with a rust pad had it looking pretty fresh.

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Rusty, Beater 2001 X5 5spd

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2017 VW GTI
2001 E46 Wagon (manual of course!)
2000 VW GTI (I see a pattern emerging)
a Saab, and a couple Volvo's thrown in for fun

steve at ogracing dot com