The default datatype in MATLAB and Simulink is double (the Step block outputs real signals of type double, as stated in the documentation). Most blocks allow you to configure the output datatype (although the Step block doesn't). You can use a Data Type Conversion block to convert it to a different type, or construct a simple step signal manually by using a Unit Delay block with a Constant (=1, specify output datatype) block as its input, and it's initial value as zero.
You can turn on display of signal dimensions from the menu Format>Port/Signal Displays>Signal Dimensions. If no dimensions are displayed, it is a scalar signal.
