Whatever you build it won't be as large as you had anticipated, consider that work benches, shelving etc. take up 2 - 3' of useable floor space. IMHO 24 x 28 is a minimum but ideally 28 x 32. Hang onto your hat and wallet as you discover current material prices..... 2 x 4 x 8 = $6.50 and 1/2 CDX sheathing $38 with the cheaper 7/16" OSB board at $28 a sheet. 1/2" sheetrock is now a whopping $12. Back in the day I was a Yard Forman for a large Lumber company: 2 x4 x 8 = $.99 1/2 CDX $7.99 7/16 OSB $5.25 and 1/2 Sheet rock $1.25.