In a nutshell, your closet is 100x60x190cm or approximately 3 feet x 2 feet x 6 feet which equal 72 square feet of surface area.
You have six walls; 2 are 2'x6', 2 are 3'x6', and 2 are 2'x3', the square footage is 2x(2x6) = 24sqft, 2x(3x6) = 36sqft, 2x(2x3) = 12sqft, so 24+36+12 equals 72 total square feet for your six sided box.
If your box is made out of 1inch (25mm) lumber then it has an R factor of 1 (R-1) if it has 3.5 inches of fiberglass insulation then it has an R factor of 13 (R-13).
To determine the heat loss you need to know the temperature differential on both sides of the box, if the inside is to be kept at 75F with the outside being 5F then the difference is 70 degrees.
Multiply the square footage by the temp difference;
72 square feet times 70 degrees equal 5040 BTU's, now divide that by your insulation factor.
5040 divided by R-1 = 5040 BTU's, 5040 divided by R-13 = 388 BTU's. BTU's divided by 3.4 equal watts - 5040/3.4 = 1482 watts of heat at R-1, and 388/3.4 = 114 watts of heat needed to maintain a box temperature of 75F when it is 5F outside at R-13.
Now that we know those numbers we still need to know what the insulation factor of your box is to determine the size of the heater necessary to keep a certain temperature. Also, any type of air infiltration either by cracks or forced air ventilation will negate much of the heat you infuse.
At my electric rate to keep a completely closed box that size with R-1 at 75F when 5F outside would cost me about $100 a month. At R-13 it would only cost $8 a month. As an FYI, 72 square feet of R-13 costs about $40.
Now to dispell some puffery, all electric heaters, be them $15 milk house heaters or the $500 Amish ceramic heaters, if they are electric then they are 100% efficient and both cost the same to operate at the same BTU levels... period. The EcoHeaters you mentioned look nice and are just a bit on the pricey side but would have no problems with using them.
Personally, I use 200 watts (700btu) heaters from a big box store that cost about $10, they don't have a thermostat but neither does the ecoheaters. The nice thing about the ecoheaters is that they put out a lower more gentle heat, either way, you'd need a controller like the Inkbird ITC-308 to control the temperature.
