Electronic – Texas Instruments Step-Down DC-DC Converter orientation

The LM2596 is probably a bit better than what you think. If you look at figure 8 in the data sheet: -

It tells you that for a 1A load (at 25ºC) the drop-out is about 0.9V. Also note that the output is now just slightly out of regulation (Vout = Vreg - 50mV).

If you are running less than 1A this figure will improve.

However, take note of figure 11: -

It is telling you that at 25ºC the minimum operating supply voltage is about 3.6V (and this id for producing a miserly 1.23V on the output).

There are better devices than this I suspect and you need to consider what your maximum load current is when selecting one.

The LP2950 is a linear regulator and if you look at figure 10 it tells you that for a 50 ohm load and 4V inputted you will get about 3.6V coming out. Also look at figure 12 because this gives a different viewpoint in terms of output current.

To fully interpret this drawing, it's useful to know a bit about GTD (Geometric Dimensioning and Tolerancing).

In answer to your specific question, the numbers refer to the maximum (top) and minimum (bottom) dimensions. So the leg will be somewhere between 0.15mm and 0.3mm wide.

The box around a dimension (not this one) indicates that it's a nominal dimension (such as the pitch 0.65mm) or what is called a "basic dimension" in GD&T. The positional tolerance of the legs is specified by the box with the 0.13mm in it and refers to maximum material condition (beyond the scope of this answer, but see the link if you're interested). In essence it allows the leg to be more out of the position dictated by the nominal pitch if it's thinner, so a 0.15mm wide leg can be more out of position than a 0.3mm wide leg. That allows the manufacturer to minimize costs, yet still retaining what matters to the user (the actual metal of the leg should fall within the target).

I agree with others that you should be able to find a suggested footprint and that this is a standard package (though not SO8 given the relatively fine pitch).

Edit: This is a DCT package, here is TI's recommended footprint for it (from the TXS0102 datasheet):