If it's CTC - 51cm, I suspect it is a large. Cotic listed the Roadrat with the seat tube measured C-T. Geo's here: https://cotic.co.uk/product/RR3#sizing
@Bearlegged started
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If it's CTC - 51cm, I suspect it is a large. Cotic listed the Roadrat with the seat tube measured C-T. Geo's here:
https://cotic.co.uk/product/RR3#sizing