The most important difference between long /i;/ and short /I/ is not that one is long and the other is short. It is true that the long /i;/ in "feel" (= sentir) is longer than the short /I/ in "fill" (= llenar), but that only happens when the consonants after the vowel are the same, as in these cases:

Long /i;/	Short /I/
meal (= comida) steal (= robar) / steel (= acero) wheel (= rueda) feet (= pies) seat (= asiento) sheep (= oveja)	mill (= molino; fábrica) still (= todavía) will (= auxiliar para el futuro; voluntad) fit (= ajustar, encajar) sit (= sentarse) ship (= barco)

However, a vowel can become slightly longer or shorter depending on the consonants that come after it. Stop consonants (/p/, /t/, /k/) make it shorter, while consonants like /m/, /n/, /l/ and especially /r/, make it longer (however, an "i" followed by an "r" is pronounced /3;/, as in "girl").

Thus, the long /i;/ in beat (= ritmo; latir) is as short as the short /I/ in bill (= factura), because the final /t/ makes the long /i;/ shorter, while the final /l/ makes the short /I/ longer.

The real difference then is not in the length, but in the quality: when pronouncing a long /i;/, your lips are more closed and spread wider (that is why English people say "cheese" when they want to appear with a big smile in a photo), so it is very similar to a Spanish stressed "i" like the one in "hilo".

In contrast, when pronouncing a short /I/ your lips are more relaxed, forming a similar shape to that used to pronounce the "e" (this sound is very similar to the schwa sound, and sometimes both are possible and correct), so it is more similar to a weak Spanish "i" like that in "rápido".

You can easily guess if a word must be pronounced with a long or short sound in most cases:

Long /i;/ is usually spelt with:	Short /I/ is usually spelt with:
A double vowel, as in "street", "team", "receive", "piece", "people", "Caesar".	An "i" followed by a consonant: "it", "his", "fit", "permit", "kitchen", "picture", "myth". (Remember that "ir" is usually pronounced as /3;/, as in "girl" /g3;l/ or "miniskirt" /%mInI"sk3;t/). When it is not clear whether the consonant is in the same syllable, a double consonant indicates that the "i" is short: brilliant, winner, fitted; compare how we pronounce "write" and "written" (hence the rule about doubling the last consonant after a single vowel: compare how we pronounce "writing" and "hitting"). An "y" followed by a consonant can also be pronounced as a short /I/: bicycle /"baIsIkl/ However, both "i" and "y" are often pronounced as /aI/, and there are few or no rules to help us know whey they should be pronounced as /aI/ or /I/. cycle /"saIkl/ motorcycle /"m@Ut@saIkl/
A stressed "e", as in "be", "he", "she".	An unstressed "e": "because", "become", "repair", "remember" However, a word beginning with re- can also be pronounced with an /e/, as in respresent /reprI"zent/, reservation /ˌrez@rˈveɪʃ@n/, but reserve /rI"z3:v/.
Stressed "ey", as in "key", "keyboard" (but "ey" can also be pronounced as /eI/, as in "hey" /heI/ or "obey" /Q"beI/).	Unstressed "ey" is pronounced /i/: "money", "monkey", "honey", "kidney" (1)
An "e" followed by a consonant and another "e": "complete", "delete", "athlete".
	"ui" (stressed or not), the same as in Spanish "guitarra": "guitar", "biscuit", "building", "circuit"

(1)

In recent years, linguists have (more or less) agreed that a final unstressed -i is pronounced in a more clear way than a short /I/ and so it should be transcribed as an /i/, as in "lucky" /"lVki/, which makes it the same sound as the Spanish "i" in "Javi": it is pronounced with the same tongue and lips position as /i;/, but it is as short as /I/. Here are some examples of words containing one or more "i" sounds: pretty /"prIti/ completely /k@m"pli;tli/ infinitely /"InfIn@tli/ (Br Eng), or /"Inf@nItli/ (Am Eng) (in the Cambridge Dictionary), or /"InfInItlI/ (other dictionaries)