It's "tall" being it's bigger than a pony, but it's on the small end for a horse.
1-2	It's	_	_	_	_	_	_	_	_
1	It	it	PRON	PRP	Case=Nom|Gender=Neut|Number=Sing|Person=3|PronType=Prs	4	nsubj	4:nsubj	_
2	's	be	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	4	cop	4:cop	_
3	"	"	PUNCT	``	_	4	punct	4:punct	SpaceAfter=No
4	tall	tall	ADJ	JJ	Degree=Pos	0	root	0:root	SpaceAfter=No
5	"	"	PUNCT	''	_	4	punct	4:punct	_
6	being	be	AUX	VBG	VerbForm=Ger	9	cop	9:cop	_
7-8	it's	_	_	_	_	_	_	_	_
7	it	it	PRON	PRP	Case=Nom|Gender=Neut|Number=Sing|Person=3|PronType=Prs	9	nsubj	9:nsubj	_
8	's	be	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	9	cop	9:cop	_
9	bigger	big	ADJ	JJR	Degree=Cmp	4	advcl	4:advcl	_
10	than	than	ADP	IN	_	12	case	12:case	_
11	a	a	DET	DT	Definite=Ind|PronType=Art	12	det	12:det	_
12	pony	pony	NOUN	NN	Number=Sing	9	obl	9:obl:than	SpaceAfter=No
13	,	,	PUNCT	,	_	20	punct	20:punct	_
14	but	but	CCONJ	CC	_	20	cc	20:cc	_
15-16	it's	_	_	_	_	_	_	_	_
15	it	it	PRON	PRP	Case=Nom|Gender=Neut|Number=Sing|Person=3|PronType=Prs	20	nsubj	20:nsubj	_
16	's	be	AUX	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	20	cop	20:cop	_
17	on	on	ADP	IN	_	20	case	20:case	_
18	the	the	DET	DT	Definite=Def|PronType=Art	20	det	20:det	_
19	small	small	ADJ	JJ	Degree=Pos	20	amod	20:amod	_
20	end	end	NOUN	NN	Number=Sing	4	conj	4:conj:but	_
21	for	for	ADP	IN	_	23	case	23:case	_
22	a	a	DET	DT	Definite=Ind|PronType=Art	23	det	23:det	_
23	horse	horse	NOUN	NN	Number=Sing	20	obl	20:obl:for	SpaceAfter=No
24	.	.	PUNCT	.	_	4	punct	4:punct	_
