First off, congrats on blooming a roth, that’s a huge achievement and something to celebrate! Any unbloomed seedling is going to be a risk, there are a range of possible outcomes from exceptional and better than the parents, to small and/or deformed. The average size with modern breeding is usually in the 23-26cm range, but good plants will be 30cm+. A lot has to do with the age, maturity, and health of the plant. As the plant gets bigger and stronger the flower size and quality will improve, but a 12cm roth is unlikely to ever be a 25cm roth. Even crosses from the largest flowered plants can have some smaller flowered offspring. TON has some huge roths, the most famous of which is ‘Tarantula’, it was awarded with a 39cm flower, I believe. I’m not sure if it consistently flowers that size, I would guess not, so even the same plant can be variable year-to-year. Any of the latest breeding from TON, Orchid Inn, or Paph Paradise should be capable of producing offspring with 30cm flowers, but the only way to guarantee getting one of those plants would be to buy it as a select plant in bloom.