I don't know if there's any regulation regarding how big the brake lights should be, but I'd bet the i-Pace adheres to it.
There are photometry standards. Here's the table from the FMVSS 571.108.
S7.3.13.1 Each stop lamp must be designed to conform to the
photometry requirements of Table IX, when tested according to the procedure of S14.2.1, for the number of lamp compartments or individual lamps and the type of vehicle it is installed on.
Table IX: Stop Lamp Photometry Requirements
1 | 2 | 3 | 1 | 2 | 3 | | | | |
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Lighted Sections | Lighted Sections | | | | | | | | |
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Group Number | Test Point (degrees) | Minimum Photometric Intensity Ratio Where Combined With A Tail Lamp(5) | Minimum Photometric Intensity(1)(2) (cd) | Group Minimum Photometric Intensity (cd) | | | | | |
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1 | 20L | 5U | 3 | 10 | 12 | 15 | 50 | 60 | 70 |
---|
5D | 3 | 10 | 12 | 15 | | | | | |
5L | 10U | 3 | 16 | 19 | 22 | | | | |
10D(4) | 3 | 16 | 19 | 22 | | | | | |
2 | 10L | 5U | 3 | 30 | 35 | 40 | 100 | 115 | 135 |
---|
H | 3 | 40 | 47 | 55 | | | | | |
5D | 3 | 30 | 35 | 40 | | | | | |
3 | V | 5U | 5 | 70 | 82 | 95 | 380 | 445 | 520 |
---|
5L | H | 3/5(6) | 80 | 95 | 110 | | | | |
V | 5 | 80 | 95 | 110 | | | | | |
5R | 5 | 80 | 95 | 110 | | | | | |
V | 5D | 3 | 70 | 82 | 95 | | | | |
4 | 10R | 5U | 3 | 30 | 35 | 40 | 100 | 115 | 135 |
---|
H | 3 | 40 | 47 | 55 | | | | | |
5D | 3 | 30 | 35 | 40 | | | | | |
5 | 5R | 10U | 3 | 16 | 19 | 22 | 50 | 60 | 70 |
---|
10D(4) | 3 | 16 | 19 | 22 | | | | | |
20R | 5U | 3 | 10 | 12 | 15 | | | | |
5D | 3 | 10 | 12 | 15 | | | | | |
Maximum Photometric Intensity(4) | 300 | 360 | 420 | | | | | | |
---|
(1) The measured values at each test point must not be less than 60% of the minimum value.
(2) The photometric intensity values between test points must not be less than the lower specified minimum value of the two closest adjacent test points on a horizontal or vertical line.
(3) The maximum photometric intensity must not occur over any area larger than that generated by a 0.5° radius within a solid angle defined by the test point range.
(4) Where stop lamps are mounted with their axis of reference less than 750 mm above the road surface, photometry requirements below 5° down may be met at 5° down rather than at the specified required downward angle.
(5) When a taillamp is combined with a stop lamp and the maximum luminous intensity of the taillamp is located below horizontal and within an area generated by a 0.5° radius around a test point (1.0° radius on lamps installed on a vehicle 2032 mm or more in overall width), the ratio for the test point may be computed by using the lowest value of the taillamp luminous intensity within the generated area.
(6) Values followed by a slash (/) apply only to lamps installed on multipurpose passenger vehicles, trucks, trailers, and buses of 2032 mm or more in overall width.
For more information reference
49 CFR § 571.108 - Standard No. 108; Lamps, reflective devices, and associated equipment.