It's ~792 KiB.  At that file size and that protection, the number of nodes is irrelevant.  The layout consists of a single stripe, with 3 full 128 KiB data stripe units (384 KiB) and 1 partial data stripe unit (126 KiB).  The FEC calculation using these stripe units produces 2 full FEC stripe units (256 KiB).  So at a bare minimum you have:   384 + 126 + 256 = 766 KiB  There's also metadata associated with files in OneFS, things like inodes, extension blocks, etc.  At that file size, for example, OneFS has created an extension block (used to store block addresses) off each inode, so those alone account for another 24 KiB (at +2:1).  The inodes themselves also consume space, but they are allocated (today) in 512 B chunks within an 8 KiB block.  So that's another 1.5 KiB  for the three inodes (at +2:1).  So with the basic stripe unit calculation  (766 KiB) and a basic estimate of metablock overhead (24 KiB) and the space for inodes (1.5 KiB), you're at about 792 KiB...
N+2:1, 7 nodes cluster. file size - 510KiB, how much space? Can you please check my answer and let me know either way and explain if it is wrong. Thanks.  My answer - 768KiB
space calculation on Isilon