Hi Venkat.g -- Not sure if SI means Signal Integrity, or you mean Si as in Silicon. No problem for Si, any thickness or thinness, any conductivity.
For tools like Sonnet, what matters is that cell size be small compared to wavelength. Usually no larger than 0.05 wavelength. Even smaller if very high accuracy is required. The actual physical size does not matter. Only the size with respect to wavelength is important.
Tools like Sonnet are usually intended for high accuracy. If you are doing Signal Integrity, 5 - 10% error is often OK. If we keep the transmission lines subsectioned one cell wide, the analysis is much faster, and error is on the order of 5%, so it can be used.
If each layer is planarized prior to the next layer being added, a planar tool like Sonnet is fine. If the conductors ride up and down over dielectrics and conductors on lower layers, this is more likely to require a volume meshing tool.
Sonnet uses MoM, which divides a circuit into subsections, then fills a big matrix with the couplings between each subsection. This matrix is then inverted. We use only non-iterative matrix solve. This means you are limited to about 20,000 subsections with 2 GBytes of RAM. This takes about 20 minutes to invert on a 4 GHz PC. If you go to single precision, you can do about 30,000 subsections. Thus circuit size will be a limiting factor. We will have a 64 bit version this summer, so that limit will go away. There are some tools that use iterative matrix solvers. These tend to fail now and then, so we do not use them, but they are available.
Another problem you might encounter is that the EM tools do not include the transistors. You have to leave ports for them. If there are 1000 transistors, this is no problem (for the full version of Sonnet, deembedding turned off). If there are 1,000,000 transistors, it is not possible.
SonnetLite is limited to 16 Mbytes (about 1500 subsections), three dielectric layers, two metal layers (including vias) and four ports. There is no time limit. You have it forever. You can certainly easily try out some simple circuits, like parallel coupled lines and cross-overs. Have fun!