Field size constrains spatial and temporal management of agriculture with implications for farm profitability, field biodiversity and environmental performance. Large, conventional equipment struggles to farm small, irregularly shaped fields efficiently. The study hypothesized that autonomous crop machines would make it possible to farm small non-rectangular fields profitably, thereby preserving field biodiversity and other environmental benefits. Using the experience of the Hands Free Hectare (HFH) demonstration project, this study developed algorithms to estimate field times (hr/ha) and field efficiency (%) subject to field size in grain-oil-seed farms of the United Kingdom using four different equipment sets. Results show that field size had a substantial impact on technical and economic performance of all equipment sets, but autonomous machines were able to farm small 1 ha non-rectangular (i.e., right-angled triangular) fields profitably. Small fields with equipment of all sizes and types required more time, but for HFH equipment sets (i.e., 28 kW conventional machine with human operator and 28 kW autonomous equipment set) field size had the least impact. HFH linear programming model solutions show that autonomous machines decreased wheat production cost by £21/ton to £40/ton for small non-rectangular fields, while 112 kW and 221 kW equipment sets with human operators were not profitable for small fields. Technical and economic feasibility in small fields imply that autonomous machines could facilitate biodiversity and improve environmental performance.