Double　emulsion　droplets　(DEs)　are　water/oil/water　droplets　that　can　be　sorted　via　fluorescence-activated　cell　sorting　(FACS),　allowing　for　new　opportunities　in　high-throughput　cellular　analysis,　enzymatic　screening,　and　synthetic　biology.　These　applications　require　stable,　uniform　droplets　with　predictable　microreactor　volumes.　However,　predicting　DE　droplet　size,　shell　thickness,　and　stability　as　a　function　of　flow　rate　has　remained　challenging　for　monodisperse　single　core　droplets　and　those　containing　biologically-relevant　buffers,　which　influence　bulk　and　interfacial　properties.　As　a　result,　developing　novel　DE-based　bioassays　has　typically　required　extensive　initial　optimization　of　flow　rates　to　find　conditions　that　produce　stable　droplets　of　the　desired　size　and　shell　thickness.　To　address　this　challenge,　we　conducted　systematic　size　parameterization　quantifying　how　differences　in　flow　rates　and　buffer　properties　(viscosity　and　interfacial　tension　at　water/oil　interfaces)　alter　droplet　size　and　stability,　across　6　inner　aqueous　buffers　used　across　applications　such　as　cellular　lysis,　microbial　growth,　and　drug　delivery,　quantifying　the　size　and　shell　thickness　of　＞22　000　droplets　overall.　We　restricted　our　study　to　stable　single　core　droplets　generated　in　a　2-step　dripping-dripping　formation　regime　in　a　straightforward　PDMS　device.　Using　data　from　138　unique　conditions　(flow　rates　and　buffer　composition),　we　also　demonstrated　that　a　recent　physically-derived　size　law　of　Wang　et　al.　can　accurately　predict　double　emulsion　shell　thickness　for　＞95%　of　observations.　Finally,　we　validated　the　utility　of　this　size　law　by　using　it　to　accurately　predict　droplet　sizes　for　a　novel　bioassay　that　requires　encapsulating　growth　media　for　bacteria　in　droplets.　This　work　has　the　potential　to　enable　new　screening-based　biological　applications　by　simplifying　novel　DE　bioassay　development.　©　2022　The　Royal　Society　of　Chemistry.