Hi,
What matters for VTT is the file size (the bigger the more time it takes to load) and playability. Grids are placed to interact with the the game mechanics of the software and are useful for battlemaps, to compute how many squares a player or NPC can move in a turn/round, distances, blast areas of spells for instance. The scale is usualy 5' for a square, so the token of a human size character will occupy a square. I don't know for other softwares, but here is how it works in FG :
1. The GM load a map (your Terror from the deep map for instance)
2. He/she then defines the software grid position and size as here (if the map is already "gridded" he/she have to place it in accordance) :
map1.jpg
3. The software grid appears (thin lines over the map) :
map2.jpg
4. Now, the software can calculate distances, so if a player can move 30' in a round, his/her token will be able to move only 6 squares, and a blast area of a 25' square will be shown as here :
map3.jpg
5. The GM share the map with the players (map is downloaded from the GM to the players computers, this is what can take time depending on the size.
So your maps do not necessarily have to have a grid on the whole map, a square somewhere near a corner is enough for the GM to be able to place the software grid.
For the grid dimension, I think 50px per square is enough (speaking FG here, 70px for Roll20), but, as in some situations players and GMs may have to zoom in, if your map is too pixelated at 200%, you may consider to produce a 100px/square grid.
Playability : For encounters maps, a player/NPC token will occupy a square, so verify that they can freely move on your map. Your Terror from the deep is good here, as the tokens can move even in the narrow passages (there are squares to put the tokens in)
For the dimension, it really depends on the type of encounter : It is not necessary to have a 100*100 squares map for a encounter between 5 adventurers and 6 goblins for example, unless the GM has planned some pursuits/racing around the map perhaps.
To resume: Choose the number of squares according to the encounter type and number of tokens, remember that a square=5', choose the grid size (50, 70, 100) depending of the map and the size of the file (it is perhaps better to work with 100px and to shrink after, although I know that 7/10 is not a round number), save your final map at the lowest jpeg quality you can afford.
Hope this helps,
Regards