You don't need to worry about height width relationship with dwarfs as I don't know of any tank that is lower than 3". Full size dwarfs run around 3/4" to an inch and you can put 3 or 4 on a silver dollar.
Buy a tank bearing in mind how many you wish to keep. A 5g tank will mean if you only have two or three pairs they will be difficult to see.
The biggest reason people leave the dwarf keeping is due to getting fed up with the hatching/enriching/feeding of the live brine shrimp nauplii to the dwarfs.
I'd suggest before you proceed with the dwarf keeping hobby, to buy some brine shrimp cysts and start hatching them so you get an idea of the workload you are going to be doing.
First you hatch, then harvest and rinse so they can be enriched, and then fed to the dwarfs about 3 times a day (the feeding part). You CAN hatch a larger batch to get you through about 5 to 7 days, but you will still need to enrich them for each feeding because the egg sacks quickly deplete and are not sufficient then for dwarf food. You can make a cheap simple
HATCHER/ENRICHER with two litre pop bottles.
Unfortunately no one has succeeded in getting dwarfs onto non live foods for any duration so live brine is the easier of the options that do work. Pod forms are a great addition to the brine feeding but are almost impossible to provide sufficient for pods as the only source of food.