How do I ensure image built with ImageBuilder is not too big for my hardware

The binwalk utility can provide some insight as to how large the overlay file system is. It's no guarantee that it won't brick your unit, but at least a bit of insight into if the flashed ROM will likely have enough storage to support configuration and operation.

Here's an example that I have running on my Archer C7:

$ binwalk out/OpenWrt-2018-09-05_2218-0700-ath79-generic/targets/ath79/generic/OpenWrt-2018-09-05_2218-0700-ath79-generic-tplink_archer-c7-v2-squashfs-sysupgrade.bin 

512           0x200           LZMA compressed data, properties: 0x6D, dictionary size: 8388608 bytes, uncompressed size: 5034275 bytes
1587612       0x18399C        Squashfs filesystem, little endian, version 4.0, compression:xz, size: 10097010 bytes, 1319 inodes, blocksize: 262144 bytes, created: 2018-09-02 18:58:29

size: 10097010 bytes -- About 10 MB for the ROM file system on this Archer C7 (16 MB flash)

and the output of df on the running device

jeff@office:~$ df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 9984      9984         0 100% /rom
tmpfs                    62248       420     61828   1% /tmp
/dev/mtdblock4            4736       860      3876  18% /overlay
overlayfs:/overlay        4736       860      3876  18% /
tmpfs                      512         0       512   0% /dev

You'll typically want at least three "erase blocks" in your overlay filesystem for it to be usable.