French TGV in a canal

The report said that there were seven people in the cab of the TGV at the time of the accident. The authors suggest that this may have impaired the driver’s sight lines, and hindered the ability of a second driver to intervene to slow the train.

Just how big is the TGV cab interior?

I wondered that. From photos it it would appear to be not that big, although not many show the back of the cab. 'le machiniste' sits in the centre, with control consoles surrounding them.
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The size and layout of the cab depends on the actual model of TGV - they've been around for 30 years in many iterations, after all. The model involved was a TGV Duplex Dasye, but equipped with POS heads/engines, i.e. different ones than used normally with the waggons (the original heads of the involved TGV No. 744 were reused for assembling TGV No. 750 as far as i know; TGV No. 744 had different heads to also operate under German 15 kV lines). TGV No. 744 had been on a test run campaign of the LGV EE since end of September, with 200 runs across the 106 km new-built route planned.

Older TGV models have the machiniste seated off-center to the left, with a small extra seat on the right and slightly more room on the sides. The cabin in those older models can fit two sitting plus about five standing behind them.

The cab of the POS head supposedly has an allowed capacity of four (while driving), two in the cabin were apparently drivers - not unusual for a test run. However, during shows with the train standing where you can go inside the cab of POS heads SNCF usually sets a limit of five people at a time. Assume that that does not include a sixth person watching over that group and enough room for them to shuffle around a bit in the cab.