I read somewhere that an actual bomb requires enrichment quite a bit higher than 90% ...
Nope. You can use lower enrichment with no problem, as long as you use the right component design (gun-type or implosion) and mass of enriched uranium.
The Hiroshima "gun-type" bomb used 64 kg of uranium. 50 kg were enriched to 88%, and 14 kg were enriched to 50%. The average enrichment was just under 80%.
The critical mass for 85% enriched uranium is approximately 50 kilograms / 110 lbs. This is a sphere about 6-3/4" in diameter.
The Hiroshima bomb, as a gun-design, was not even tested prior to dropping. The design concept was deemed so reliable it was considered guaranteed to work.
Approximately 21 to 30 ounces (est.) of the core mass actually converted into energy in the explosion. The explosive force was equivalent to 32,000,000 lbs of TNT - (16 kt = 32 million pounds).