Nigel Sadler published a diagram in 2009 showing the relative sizes of the Thunderbirds. They’re difficult to compare generally as Thunderbirds 3 & 5 are never really shown next to Thunderbirds 1, 2 & 4. Likewise, Thunderbird 1 is never really shown next to Thunderbird 4 so it’s difficult to compare their sizes too. Also, Thunderbird 1 relative to Thunderbird 2 is not exactly clear (as shown in the diagram)
It’s interesting to see how huge Thunderbird 5 would be especially compared to Thunderbird 4!