Can I make graphs on which symbol sizes correspond to the sample size (N)?

This is called a "bubble chart" or "bubble plot", and is available within Prism as a built-in graph type for data entered into the multiple variables data table. Creating a graph using data in a multiple variables data table is a little bit different than making graphs from other table types within Prism. With other graph types, the location of data (symbols) on the graph is determined by where the data are entered on the data table. In contrast, for multiple variables graphs (including bubble plots), you select which variables you want to assign to the X and Y axes (selecting one variable for each axis). This will determine the location of the data on the graph. However, multiple variables graphs also allow you to select additional variables to specify the symbol color and/or size. To create a graph with symbol size corresponding to sample size, simply enter the values for the X-coordinates of the symbols in one variable, the values for the Y-coordinates in a second variable, and the sample size in a third variable (be sure that the values for X-coordinate, Y-coordinate, and sample size for a single symbol are all on the same row). Then assign these variables to the graph accordingly.

There are also ways that this sort of appearance can be achieved with other graphs within Prism, but it requires a work-around. For graphs other than multiple variables graphs, you can only control symbol size for an entire data set, not individual points. But you can superimpose lots of data sets on one graph. So make each point represent a different data set, and plot all on one graph. The easiest way to do this is to use a single data table. Enter the X values all in one column (the first, "X" column). Then enter each Y value in a different column -- the first in column A, the second in the second row of column B, and so on. Then go to the graph, and bring up the "Format Symbols and Lines" dialog. Use the same shape and color for each data set (point), but adjust the sizes depending on your needs.