I can work out the scale of the ships if you name them.
For instance:
1. A PLAN Type 051 Luda DDG is 132 meters long. A 30cm long model would be 1:440.
2. A PLAN Type 052 Luhai DDG is 153 meters long. A 30cm long model would be 1:510.
3. A Russian Sovremenny DDG is 156 meters long. A 30cm long model would be 1:520.
4. A USN Ticonderoga CG is 172 meters long. A 30cm long model would be 1:576.
5. A USN Nimitz CVN is 330 meters long. A 30cm long model would be 1:1100.
Results: Most differences in scale are small unless the ship being modeled is very big.
At:
1/500
1/700
1
Luda
26.4cm
18.8cm
2 Luhai
30.6cm
21.8cm
3 Sovremenny 31.2cm 22.3cm
4 Ticonderoga 34.4cm 24.6cm
5 Nimitz
66.0cm
47.1cm