Best Practices when dealing with big surfaces

Since you can use reference surfaces just as easily as those in the active file, it's better to break them up into logical portions. I can't recall exactly what the optimal size is, but I know it's better to have several smaller ones as opposed to one big one.

Okay Thank you 

I've found that keeping dgn surface files and/or 3D model files under 10mb has the most efficient workflow. It's a good target, but you shouldn't worry too much if they get up to 12mb or so. As MaryB mentioned, usually a mile section segment is small enough, unless there are several template points and design intent utilizing the existing surface (e.g. milling and resurfacing, RRR projects). 

12 Mb okay I am definitely above the limit, my file is 1,106 Mb. thanks for the advice does that mean that I should split my surface into 10 then? 

Is it a very dense model or a large area of coverage?

Usually large areas do not need the same level of detail needed for fine design. A thinned surface will be smaller.

Even if it is a small area there are often areas where a reduction of points will not impact the accuracy of the model. As much power todays computers have, data management is still something to be considered.