I had a very small IRA. That was substantially increased when I inherited my spouse's IRA. I rolled over a majority of that to a ROTH IRA. Is the rollover taxable ?

Yes, if it was a traditional IRA rolled over to a Roth it would be taxable.

You can convert all or part of the money in a traditional IRA into a Roth IRA. ... You will owe taxes on the money you convert, but you'll be able to take tax-free withdrawals from the Roth IRA in the future.