Finding MOSFET Spice Model with Specific Length and Width – MOSFET, Circuit Design, LTSpice, SPICE

The model will probably support whatever you throw at it. After all, a model is only a fancy mathematical equation - there are not usually any constraints. Without any particular context, such as a manufacturing process, there really isn't a good way to figure out an upper limit.

If you got the model from a design kit, then there would be boatloads of documentation on what specific structures the model fit was based on, and the boundaries at which the model no longer accurately portrays the device.

Since you have neither the documentation or the process that goes along with it, I can suggest some boundaries for length and width for reasonable design.

Minimum Length:   0.25 um
Maximum Length: 100.00 um
Minimum Width:    0.25 um
Maximum Width:  100.00 um

Note that you can create a larger effective width by simply adding more copies of the transistor. You can do this through the model by declaring the number of fingers, or the multiplicity of the device. This lets you get larger transistors without the downfalls of really wide gates. For a longest effective width, I'd probably say the biggest I've seen on an IC is 2,000um to 5,000um for LNA input transistors. I don't think you should need anything so large (unless you're designing an LNA).

The precise method depends on the particular spice engine.

Most have something called a subciruit or package definition that would let your create the component as a schematic.

Many also have the ability to define the behavior of an element programmatically. By entering a formula or script, relating the various values.

There are many different levels of FET transistor models. It is beyond the scope of this answer to describe them. A large number of them are described in this HSPICE manual, the manual of your own particular spice implementation will likely has similar descriptions of the models it supports.

To determine reasonable values for the parameters, you are going to need to look at the datasheet for the component, which you should be able to download from the manufacturer.

You may in-fact be able to download a existing spice model for the component from the manufacturer -- or from the website of a spice engine -- though your might have to convert it to the format preferred by your spice engine.