The zone was originally to be 221km2, for a capacity of at least 1.7GW.
Following industry protests at the size of the zone, however, De Backer has now added an additional 60km2.
"This means the 1.7GW of wind capacity can be built on a larger surface area, which has a favorable influence on the development price," De Backer said.
While De Backer's proposal falls short of the 400km2 the industry called for, "it is better than before" said Annemie Vermeylen, secretary general of the Belgian Offshore Platform.
With fewer, larger turbines and other innovations, in future developers may be better able to manage the impact of wake effects and maximise installed capacity, she added.
The minister is also working on a new financing system, designed to drive down costs, and a new tender process whereby the winners will be awarded the concession, environmental permit and a confirmed grid connection.
This will offer the developers greater legal certainty and help reduce costs, Vermeylen said.
"I am convinced that in time we will be able to build wind farms at sea without subsidies," De Backer added.
The priority now is to ensure speed of deployment. The industry is calling for the new offshore tender to be launched early 2020, at the same time as the new Marine Spatial Plan comes into force.
The timetable "is ambitious but not impossible," Vermeylen said.
SeaMade
In a separate move, Otary, Engie Electrabel and Eneco Wind Belgium have reached financial close on the combined 487MW SeaMade project.
SeaMade comprises the last two projects remaining under Belgium's first offshore allocation.
The European Investment Bank is providing a loan of €250 million towards the costs, with additional support from the Danish export credit agency DKF and 15 commercial banks.
Siemens Gamesa Renewable Energy will supply 30 SG 8.0-167 DD turbines to the 252MW Seastar area and 28 units to the 235MW Mermaid concession.
Engie Fabricom, Tractebel, Smulders and Geosea will be responsible for the engineering, procurement, construction and installation (EPCI) of the two offshore substations.
These will be connected to the Modular Offshore Grid, currently under construction.
Hellenic Cables will supply the export cables to be installed by Tideway, while Dredging International has been handed an EPCI contract for the monopile foundations and inter-array cables.
Offshore construction will start next summer, for commissioning in 2020.
The decision comes less than two months after an FID was taken at the 224MW Northerwester 2 project, developed by Parkwind and to be supplied with MHI Vestas turbines.
