mesh creation

I have a question regarding mesh creation. I am working with Petrasim, T2VOC. I appreciate if you could help me. Is there any limitation for creating mesh?

I know in finite difference software like MODFLOW , for example, grid spacing shouldn't increase by more than 50 percent between adjacent cells or a grid cell shouldn't be more than 10 times larger in one direction than in another. 

As I have some convergence problems during my work, I think the issue may be related to large mesh size. I appreciate if you could help me with this issue or provide me a reference?