I'd generally stick to 2 GBytes, sometime less, depending on plans for further processing the wav files.
There is a fundamental limit to wav file file sizes of 4 Gigabytes. This is because the size of the data is stored as an unsigned 32 bit integer in the wav file header and this number simply can't get any bigger than 2^32 bytes. However, some languages (Java being an example) don't support unsigned integers, so are effectively limited to sizes of 2^31 which is about 2 Gigabytes (PAMGuard has a modified wav stream reader since the standard Java one fails with files > 2Gbytes).
If you plan further analysis with Matlab, when you read the files into Matlab, the default audioread(...) function converts the 16 bit integer values in the wav files to 64 bit double precision values, which use four times as much memory. Therefore reading in a whole 2Gbyte wav file into Matlab may require 8Gbytes of RAM (16 for a 4Gbyte wav file). If you've lots of RAM, this may not matter too much and there are ways around this, such as only reading part of the file. However, if you're planning to use software which will read a whole file at a time into memory, I'd recommend a much smaller file size.