Turnip Oriental, Tokyo Cross, very fast growing can be harvested any time during growing for whatever size you require.
With a maximum 35 days to harvest this splendid variety produces high yields of small, white skinned turnips with a lovely, sweet, flavour. Sow as late as September. Perfect for growing close together in small spaces as a mini veg, they don't need to be peeled for cooking and great for succession sowing.
Definitely worth growing if you've never tried them and they take very little time, energy or space. GIY grows these in veg trugs on the office balcony.