There are two factors that affect the maximum size of the Lisp image: the size of real memory, and the layout of memory. On most platforms you can relocate LispWorks to avoid clashes with other software as described in Startup relocation.
If LispWorks becomes significantly larger than the size of the real memory, then paging will be the main activity and LispWorks will not function effectively.
This is Operating System-dependent:
On Solaris, 32-bit LispWorks is mapped at #x10000000. In principle it can grow to almost #x80000000 (the libraries are at higher addresses).
For the other platforms and for 64-bit LispWorks, see the discussion in Startup relocation.
The simplest way to see the current Lisp allocation is to call (room t).
To obtain values representing the current total allocation, call room-values.
LispWorks User Guide and Reference Manual - 13 Feb 2015