Well, I claim no special insights. However, every OS has a counting limit set by the maximum word size it can handle. In a 64-bit system that amounts to 2^64 power which is about 17.5 TBs if my memory math is right (may not be 'cause I'm 70 and don't remember as well as I used to.🙂) If my memory math is wrong then it may be more like 16,000,000 TBs. In any case it's really big!
File sizes are counted in blocks using a single word to count the number of blocks (usually.) So a computer that uses a 64-bit word can count as high as one 65-bit word can handle - 2^64. A 32-bit word counts to 4 GBs hence the 4 GB file size limit on older PC systems.
I'm sure somewhere someone may have tested this, but I don't know where you might find the information. I'm not a Safari expert so I don't know if Safari itself uses a smaller word-size to count file blocks. I've never downloaded anything much larger than 8 GBs or so. I have formatted, zeroed, and used 1 TB drives. Other users have 2 and 3 TB drives installed in Mac Pros.