Set default GC card size to 251 blocks

[bodgit]

Fixes #1

[coveralls]

Pull Request Test Coverage Report for Build 3857678389

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 68.966%

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Totals
Change from base Build 3857657551:	0.0%
Covered Lines:	120
Relevant Lines:	174

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💛 - Coveralls