LM2576 may I use bigger or lower inductors

Be aware of the capacitance to ground on each pin of the digital pot. The spec you embedded in your question suggest a range in the order of about 100pF. I don't know the exact switching frequency of your proposed application but, if it were 200kHz, 100pF would have an impedance of about 8 kohms (reactive) and this might be significant compared to the feedback resistor values you intend to enact with the digital pot. This could mean that you get a highly unstable circuit (worst case scenario) or, a lot of ripple voltage (more probable scenario) or absolutely no problem at all.

There isn't enough information in the LT data sheet to call this one so I'd advise setting it up with standard resistor values to cover the range you expect and testing it by adding 100pF caps across those resistors just to see if they introduce oddball behaviour. Other than that it should work just fine.

If it does got belly-up on you there is another alternative and that is to "offset" the feedback voltage with a current sink/source thus "conning" the LT8705 into thinking it needs to raise (or lower) it's mark-space ratio. The current sink/source can be controlled via the pot you want to use and because the pot would be "isolated" from the actual feedback node the capacitance problem wouldn't be an issue. Current sink/sources can be designed with very low capacitances (sub 5pF) so this should be OK. Remember, the standard feedback resistors won't introduce more than 1 or 2 pF and a bit of track might add another 2 pF hence my concern.

In a Buck Converter the inductor ripple current is determined by (approximately): $$\dfrac{V_o.(1-V_o/V_i)}{L.f}$$

Which works out to be 6.66A for your values.

Designing the ripple current to be too high results in excessive RMS ripple losses in the inductor, output voltage ripple and bulk capacitor ripple losses (and heating). Too low a value results in a poor transient response limited by the inductor current slew rate.

A simple rule of thumb is to set the ripple current to 1/2 the max current of your design; for you that's 0.75A

From the formula you can see you only have a couple of options open to you.

Increase the switching frequency to about 800kHz which is very high for standard MOSFETs leading to excessive switching losses. Increase L to 133 \$\mu\$H (fairly large and higher DCR \$I^2R\$ losses); or a combination of both: i.e higher switching frequency and higher inductance. I would aim for about 300kHz and 47\$\mu\$H.