The first thing to make sure is that you are a "young person" who is not a steel rolling major.
Because the questions are too unprofessional and there are clear answers in the textbooks.
Based on the statement of your question, I can only recommend you a diameter range size: 300~380, for example, you choose 360.
The selection of the roll diameter of a three-roll steel rolling mill depends on the maximum variety of production, the pitch circle diameter of the rolling mill connector, the number of rolling mills, the size of the billet and other process requirements.
The above-mentioned roll diameter of 360 can be suitable for 500-600 rolling mills (you may not understand what 500-600 means), with at least three horizontal rows, and the largest varieties are 16# corner, 20# groove, and 20# work. , steel grade Q345. The main motor is generally around 2000W.