Print labs suggest print sizes based on pixels and not file size, because file size is not a reliable way to evaluate image quality, especially for a lossy compression format such as JPEG.
In addition, late model iPhones might store original iPhone camera photos as HEIC, and export as JPEG, resulting in two different file sizes for the same pixel dimensions. File size also depends on how much JPEG compression was applied, but even that can vary because some content compresses more than others. And if the same photo had been taken with the Lightroom app camera instead, it would be saved as DNG, which would be a third different and probably much larger file size from the same camera sensor. So the only constant that would be reliable for understanding potential print sizes would be the pixel dimensions, which should be the same in all versions of the same photo.
If I want to order 8" x 10" / A4 prints, what file size should I be using?
By knowing that you want 8" x 10" / A4 prints, we can immediately know the minimum pixel dimensions that would get you there. For the images to meet the recommended 300 ppi resolution for print:
8 inches times 300 ppi = 2400 pixels
10 inches times 300 ppi = 3000 pixels
So as long as an image is at or near 2400 x 3000 pixels (7.2 megapixels), it has enough pixels for an 8 x 10 inch print at or close enough to 300 ppi.
A4 prints are slightly taller and narrower; using the same math, the optimal pixel dimensions would be 2481 x 3507 pixels (8.7 megapixels) for a 300 ppi A4 print.
As long as the iPhone camera used here captured at least 8 megapixels, there should be no problem in terms of pixel dimensions alone. Fortunately, the cameras on iPhones released in the last 5 years can capture at least 12 megapixels, so chances are, 8 x 10 or A4 prints are going to be no problem, and you will have room for cropping if you want, as long as the cropped version still meets or exceeds the pixel dimensions above.