Doing an ordering and then selecting the first item is wasting a lot of time ordering the items after the first one. You don't care about the order of those.
Instead you can use the aggregate function to select the best item based on what you're looking for.
var maxHeight = dimensions
.Aggregate((agg, next) =>
next.Height > agg.Height ? next : agg);
var maxHeightAndWidth = dimensions
.Aggregate((agg, next) =>
next.Height >= agg.Height && next.Width >= agg.Width ? next: agg);