D+ has a Suggest Parameters tool under the Settings menu, located at the main window.
The tool gets as an input, q Max (the maximum q value), and the x, y, and z coordinates of the point, P, which is most distant from the origin in the structure, for which reciprocal grids are going to be used. 
If grids are used for the entire structure, P  should be the most distant point in the entire structure. However, when the Hybrid method is used, P should be the most distant point in the part of the structure for which grids are used.  

The tool computes the distance, L, of the most distant point from the origin. 
Practically, L is the higher of the following two value: the longest dimension in the structure (if the structure is centered around the origin - twice the radius of the sphere including the structure), or the most distant point from the origin in the structure. 
The Suggest Parameters tool can then compute the suggested Grid Size for computations that use grid. 

The Grid Size, is twice the number of spherical grid shells, and is a measure of how dense the reciprocal-space grid (or lookup table) will be. It is calculated according to the length of the structure L, contributing the highest frequency oscillation to the amplitude, and the maximum magnitude of the scattering vector. 
Note that Suggests Parameters is based on the Nyquist-Shanon sampling rate, which is a rule of thumb, sometimes a denser grid is required. Further note that it is faster to compute structures (simple or complex) when their center of mass is at the origin, as smaller grid size will be required. 

The Suggest Parameters tool also returns the RAM volume (in units of MB) that is required for storing the Grid. 

It also suggests the suitable orientation average Integration Method, based on the aspect ratio of the structure and if GPU or CPU are used. Adaptive integration methods (Gauss Kronrod for CPU or Vegas Monte Carlo for GPU) are more suitable when the structure has a high aspect ratio.

The tool also suggests the number of Integration Iterations that should be used in D+, the Convergence cutoff, the number of Generated Points within the required q range, and the Update Interval, which the time in units of ms between successive computation updates. 

If the diameter of the sphere that envelopes the structure is larger than L, the x, y, and z coordinates should be such that the resulting L will be that diameter. In that case, the integration parameters may not be optimal.

When the structure in not centered around the origin these suggestions are inaccurate, more details can be found in our user's manual.
In any event, the suggested parameters are only a guide or first approximation. In practice, higher or lower values might be more appropriate for optimal results. 

At the bottom of the Suggest Parameters pane, fitting method and Fitting Parameters are also suggested. These include the maximum number of Iterations, the fitting Convergence cutoff, the cost and loss functions, their maximum Step Size between fitting iterations, and its regularization parameter Der eps. 

On the right side of the pane, there are detailed explanations of how to use the mouse in D+ to control the presentation in both 2D and 3D Graph views.