Home Style - Mirror Size

[BrownSuga]

If you have a sideboard and want a mirror above it, do you get a mirror the same length as sideboard, or say 80% of the length etc?

Is there a ratio of mirror to sideboard, that says ooooh stylish, rather than, hmmm that looks a bit odd?

oh and at the same time, how high would you go for the mirror. The mirror height of the sideboard, or again, a ratio?

(sideboard is 1.6m long, 53cm deep)

yoohoo