In context, '+' and 'k' are both examples of "linespec" . You can have at most one linespec for every group of points.
The easiest approach would be
plot3(X, Y, Z, '+k', 'MarkerSize', 10);
but you could also use
plot3(X, Y, Z, '+', 'Color', 'k', 'MarkerSize', 10);
Note that when you use name/value pairs, that all of them must come at the end of the call, and that they apply to all of the data triples, not just to the "nearest" data triple. So for example,
plot3(X, Y, Z, '+', X1, Y1, Z1, 'Color', 'k', 'MarkerSize', 10);
would apply the linespace '+' to X, Y, Z, and would apply the Color and MarkerSize to X, Y, Z as well, but X1, Y1, Z1 would use the default marker (because no linespec giving the marker and no 'Marker' name/value pair) but would use the Color and MarkerSize because those apply to all data.
