At the Harvard Task Force meeting and in other presentations, Harvard is talking a lot about the new open space in their 20 and 50 years plans for North Allston. What hasn't been discussed as much how many new students would be living here. These two issues really can't be separated, so here is a view of the whole picture.
Harvard's 20 year plan proposed 1.4 million square feet of new housing in North Allston. This is 800,000 sq ft of undergraduate housing, 350,000 sq ft of graduate housing, and 250,000 sq ft of Business School housing.
How many people would live in 1.4 million square feet of housing? Harvard estimates that there will be 590 beds in the 350,000 square feet of graduate housing. That is 600 sq ft per bed. How many people sleep in each bed is another question, but I will assume it is 1 for the sake of this estimating exercise.
1.4 million square feet of housing & 600 sq. ft per person = 2,300 additional Allston residents.
On average, Boston has 7.4 acres of open space per 1,000 residents. (Allston/Brighton is well below that, with 4.8 acres per 1,000.)
If new open space should be created at the same time new residents are added at the 7.4 per 1000 ratio, that means that 17 acres of new open space should be created to accompany the 2,300 new Allston residents that Harvard plans to add.
Harvard's press release that accompanied the 50 year plan announces an intention to "create more than 30 acres of new open space". In the 20 year plan there are 3 acres of open space in Rena Park and an unspecified amount of open space that burying Soldiers Field Road and a Science Courtyard would create.
(The estimates of square footage of housing to be constructed comes from page 16 of The Plan for Harvard in Allston executive summary. The 250,000 sq ft of Business School housing is an estimate based on the 500,000 sq ft mentioned as part of a "Harvard Business School Academic and Housing" line item.)
More information about open space in Allston/Brighton is in the Boston Open Space plan. The Allston/Brighton portion of that plan is here.