There is a lot of information on the web concerning the selection of throttle body size.  When sizing ITBs, you need to consider that each individual throttle needs to be large enough to not overly restrict air flow into its cylinder at high RPMs.  This required size is usually close to that of the intake port.  If you add the total cross-section area of each TB in an ITB setup, it will add up to an area noticeably larger that the TB size required for a single TB setup.  It is this characteristic that makes ITBs act like grossly oversized throttles at low RPMs.  This is why you want to also be careful to not select a TB size larger than you need because this will work against you at lower RPMs.  The smaller the TB, the better modulation you get at low RPMs and the higher the intake air velocity will be.  Higher intake air velocity helps air/fuel mixing and can noticeably effect performance.  EFI does not rely on the venturi effects that carbs do, but intake air velocity still effects performance.  The selection of an ITB size is then a compromise between these multiple design choices.

The Jenvey FAQ provides some recommended ITB sizes based on HP per cylinder.  This information suggests that a 40mm ITB setup would be more than sufficient on an M10 for all but a 200HP race engine.

Below are three graphs showing the throttle position required to achieve a MAP of 80%, 90% and 98% across the RPM range on the engines I have tuned.  There is one graph for each engine and TB I have worked on.  This data was taken from datalogs during normal driving.  I used 98% as the maximum because it is as close to 100% as I could get in the data and still be sure the logged throttle position was required to maintain that MAP signal.  At 100% barometric you are not sure the throttle position is just what is required, or if it is actually more throttle than required.

From these graphs you will get a general idea of the behavior of aftermarket ITBs on an engine.  The useful operating region for the TB is triangular in shape and defined by the throttle position required to reach a MAP of 100% for any given RPM.

The graphs of the 2.0L and 2.3L engines running 45mm ITBs show the effect of increased air demands on a throttle.  The graphs of the 45mm and 40mm ITBs on my 2.0L engine show the effect of throttle size on the same engine.

On the two engines I have tuned, the MAP when maintaining speed was between 80% and 90%.  You can see from these graphs that for normal driving you typically don't need more than about 20% of the throttle.  This is why a progressive throttle linkage is so important.

Throttle position data for the 45mm ITBs on the 2.3L M10.  The throttles look like a good fit for this engine.  High RPM performance is important on this engine and the throttles don't cause any real restriction all the way up to 7000 RPM.

Throttle position data for the 45mm ITBs on my 2.0L M10.  These throttles are oversized for this application.  You never need to open them more than 50% at 4000 RPM.  In fact, at 6500 RPM you only need to open them to 80% for max air flow.  The range of throttle movement needed to vary the MAP between 0% and 90% is approximately 8% to 19% within the daily driving RPM range of 2000 to 4000 RPM.

Throttle position data for the 40mm ITBs on my 2.0L M10.  These throttles are better sized for a daily driver.  The throttles do begin to slightly restrict air flow at approximately 6600 RPM.  The range of throttle movement in the daily driving region is better however.  The throttle range required is approximately 11% to 22% for the region between 0% and 90%, 2000 to 4000 RPM.  This is about a 20% increase over the 45mm throttles on this engine.