The PlayStation 1 had two megabytes of main memory, one megabyte of video memory and half a megabyte of audio memory. Squeezing everything to fit into these limits was one of the big challenges of developing a game.
As with other consoles, games were written using specialized, expensive, proprietary development kits provided by Sony, which had similar hardware to the regular console but also the ability to download software. (There seems to have also been a development kit version called 'Net Yaroze' which was cheaper, less capable and sold to hobbyists; I'm talking here about the regular kit, sold to licensed commercial developers.)
From a developer viewpoint, it would be advantageous to have more memory in the development kit, so you wouldn't have to worry about squeezing your code and data until after doing your iteration on the gameplay.
How much memory did the (standard, official) PlayStation development kit have?