If you are going to have identical protection relays as available with the existing transformer, you may refer to the same CT/VT specifications. Of course, you need to verify to make sure those who have sized those CT/VTs have not committed any mistake.
Generally, you refer the Protective relay technical manuals for the CT/VT parameter sizing formulas / information. If you know which make/model relay you pan to purchase, it is easier. Otherwise, you have to size the CT/VTs suitable for different make/model relays.