What is the cloning/packaging capacity of an HIV-1-based lentiviral vector?

Wild type lentiviruses contain ~9.7 kb of genome including both LTRs. Artificially creating a genome larger than this will result in unstable viral particles and a dramatic drop in viral titer. For recombinant lentiviruses such as those generated using Lenti-X systems, much of the viral genome has been replaced with other useful sequences such as selection markers or fluorescent proteins, but enough space remains for cloning transgenes.

The 3' LTR of pLVX-Puro ends at 5.4 kb, so ~4.3 kb of space remains for you to clone in your gene
The 3' LTR of pLVX-IRES-tdTomato ends at 6.3 kb, so ~3.4 kb of space remains for you to clone in your gene