It is the same wheelbase as the Prius and based on the Prius underpinnings, but it is longer and a bit wider and not as tall. Here are some exterior dimension comparisons ...
Wheelbase:
Prius -- 106.3
HS 250h -- 106.3
Lexus IS -- 107.5
Camry Hybrid -- 109.3
Height:
Prius -- 67.9
HS 250h -- 59.3
Lexus IS -- 70.9
Camry Hybrid -- 71.7
Length:
Prius -- 175.0
HS 250h -- 184.8
Lexus IS -- 180.3
Camry Hybrid -- 189.2
Width:
Prius -- 67.9
HS 250h -- 70.3
Lexus IS -- 70.9
Camry Hybrid -- 71.7
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This might just be "right-sized" for me personally ... a little wider and a bit longer than the Prius while riding on the same wheelbase. I would imagine EPA estimates a bit above the Camry Hybrid, but not near the Prius ... size increases and tech increases will add to the weight, plus it uses the 2.4L engine ... however it will "heat up" quicker similar to the new RX 450h which helped increase the RX hybrid's fuel economy 8% over the old RX while increasing the engine size.
I will be interested to check one out personally ... sounds like late summer 2009. One issue is it's not a hatchback ... I would imagine the trunk will be a decent size as they've probably figured out a better place to store the batteries with the new design (stand-alone) over something like the Camry Hybrid.