In　order　to　explore　far　offshore　wind　resource,　it　is　an　eloquent　pathway　to　develop　a　far　offshore　wind-hydrogen　project.　The　size　optimization　of　electrolysis　plant　is　beneficial　and　conducive　to　upgrade　wind-hydrogen　projects＇　cost-effectiveness.　This　paper　aims　to　perform　size　optimization　as　well　as　a　techno-economic　assessment　to　contribute　to　investors＇　decision-making.　Firstly,　the　efficiency　methodology　of　Proton　Exchange　Membrane　(PEM)　electrolyzer　is　deduced　in　the　presence　of　aging　and　changing　operation　powers　since　it　directly　impacts　on　the　productivity　of　hydrogen.　Secondly,　a　chance　constrained　programming　(CCP)　model　of　size　optimization　is　established　to　reach　a　benchmark　maximum　net　present　value　(NPV)　for　the　wind-hydrogen　project,　taking　into　account　random　fluctuations　of　offshore　wind　power　output　and　replacement　price　of　electrolyzer　which　is　the　key　device　of　the　hydrogen　system,　and　the　aging　of　electrolyzer.　Further,　for　the　pursuit　of　evaluating　the　influence　of　hydrogen　sale　price＇s　irregular　fluctuations　on　the　economic　feasibility,　information　gap　decision　theory　(IGDT)　is　exerted　to　assess　acceptable　hydrogen　price　ranges　from　investors＇　point　based　on　acceptable　NPVs,　taking　maximum　hydrogen　price　uncertainty　as　the　upper　layer　objective　and　minimum　acceptable　NPV　as　the　lower　layer　objective,　and　the　delicate　IGDT-CCP　prototype　is　evolved　with　CCP　constraint　conditions.　The　particle　swarm　optimization　of　stochastic　simulation　is　adopted　to　solve　both　CCP　and　IGDT-CCP　models.　Finally,　detailed　results　are　demonstrated　and　compared　through　a　study　case,　including　optimum　size　in　view　of　multi　scenarios,　economic　viability　and　the　acceptable　hydrogen　prices.　(c)　2021　Hydrogen　Energy　Publications　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.