You can find sensor size for any camera at www.dpreview.com

$I was wondering about sensor sizes in DSLR's. With Nikon's new D40 (6
$meg), Canon's 350XT (8 meg) Nikon's D 80 10 meg) and Canon's 400 (10
$meg) - are sensor sizes similar? If so, then the 6 meg Nikon would have
$larger pixels and ability for more light capture.

  They're similar.  All Nikon DSLRs have sensors with crop factors
of approximately 1.5x.  The two Canons you mention have crop factors
of approximately 1.6x.

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The sensor size of the D40/350XT/D80/D400 are all about the same size.
Nikon uses a 1.5 crop factor to the 35 mm 'standard'
Canon uses a 1.6 crop factor to the 35 mm 'standard'

Yes it is very likely that the pixels of a 6 Mp camera are lager than the
pixels on a 10 Mp camera. It depends a littlebit how much of the area can be
used for the actual pixels, but I guestimate this will be 'similar'.
Although electronics get smaller, for a 10 Mp camera also more electronics
are needed so I guestimate the pixelsize to be 6/10 of the area of a 6 Mp
camera. So they wel be less sensitive to light, but will be full at the same
time as the larger pixels. So on a per pixel base, the single pixels of a 10
Mp camera will have less dynamic range. But the dynamic range of the
complete sensor will be about equivalent.
Because there are probably some improvements in the 10 Mp camera's which or
not yet in the 6 Mp camera's the above will not be completely true. But I
expect that the improvements are not enough to compensate for the smaller
area available for the 10 Mp pixels. So compared on a pixel basis there is a
difference. Compared for the complete sensor the 10 Mp will not be worse
than the 6 Mp.

Ben

Digital Cameras: Does Pixel Size Matter?
Factors in Choosing a Digital Camera
http://www.clarkvision.com/imagedetail/does.pixel.size.matter

Table 2 at:
Digital Camera Sensor Performance Summary
http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary