The smallest company in the S&P/ASX 300 is ARQ worth around...

The smallest company in the S&P/ASX 300 is ARQ worth around $80M, but that's certain to be kicked out at the next rebalance. Above that there are 7 more under $200M (again, all likely to be removed), and another 12 under $300M. My guess is that'll be roughly where the cutoff is, so to get in IMA would need at least $300M to be in the hunt - it depends how many other non-index stocks are above it at the review date.

You can't just look at all listed securities by market cap and cut it off at the 300th, because that includes non-eligible securities (eg S&P may exclude trusts, CDIs, tightly held companies etc)